Brake Bleeding

[Ross F.]

I'm at my wits end.
I can't get my brakes to bleed.
Ok before everyone jumps on me; let me explain my nightmare.
I have done the following.....
all new lines. Master cylinder. Booster.
stainless sleeves with o-ring seals.
bench bled the MC.
motive power bled all four corners.
still soft pedal.
maybe bad MC. Plugged both sides. Firm as can be.
Attached power bleeder and cracked every fitting starting close to the MC and working my way to the calipers.
STILL SOFT PEDAL
opened bleeds rapped on calipers.
checked proportioning valve. Bled all connections again.
still soft pedal.
Couple things I noticed.
the pedal is fairly stiff and feels normal until it moves a half inch or so. Then it goes to the floor.
with the engine running repeated pumping effects idle.
it's a new booster. I checked for vacuum leaks. Can't find any. No fluid in booster.
I don't see any leak at the calipers.
Argh!

[Bill W.]

Re: Brake Bleeding

I always start with the right rear and work my way up to the Master cylinder . I also use a pressure bleeder when possible . The cars that I put silicone fluid in were harder to bleed . I did them 3 times in a two week period and the pedal was never as hard as regular fluid but it stopped good ?

[Gary V.]

Re: Brake Bleeding

Ross,
Re-check the calipers, you may have a piston hanging up in a bore which can cause the weird pedal travel.

Gary

[Steve G.]

Re: Brake Bleeding

If these calipers have o-rings and not the original style lip seals they likely don't have springs behind the pistons (or if they do they likely won't overcome the resistance of the o-ring in the bore). So what that means is that after the air has been removed the pedal will have to be applied until the pistons are all pushed out against the pad. They were likely bottomed in the bore. Until they have traveled out they will feel like there is no resistance. To check this just keep applying them, watching the fluid level, and see if you ultimately get brakes. This is the most likely problem.

If not, take 4 pairs of vice grips. Using something soft like a couple of popsicle sticks clamp them on the rubber brake lines at the 4 wheels with just enough pressure to pinch off the lines. Is your pedal now up and solid? If so (I suspect it will be because it was firm when blocked at the mc), release the grips one at a time, checking the pedal after releasing each line. If the pedal drops after you release the first one, deal with that wheel. Either you have air in it or the pistons are not extended. Deal with that wheel b4 moving on. Repeat with the rest.

This will isolate your problem.

BTW, it's not a booster problem. Engine will sound different as you vent and replace vacuum. This is normal.
Steve

[Richard M.]

Re: Brake Bleeding

Ross, Sorry a few questions first.

...What year car and what fluid type is being used?

...Is the MC new, rebuilt outside, or self rebuilt?

...Was this a repair process or did you upgrade to Power Brakes from Manual? If upgrade, is the correct MC/Booster pushrod installed?

When you tested the MC you plugged the 2 port lines and it was a hard pedal, but........If the internal seal has been damaged it may not show a problem until the piston travels past a certain point. When you plugged the lines you then prevented the travel to go to max and had the hard pedal immediately, right? So I don't feel that is conclusive to verify the MC. You did a power bleed so I doubt air is in the system. You are certain no leaks are evident.

Recently on a friend's '72, we got to the last wheel(RF) after a successful pedal bleed of the first 3 calipers. After a few pedal bleed cycles on that last one, the pedal was hard but a inch or so down it released pressure and went to the floor. The MC went bad during the bleeding operation. Prior to this work the MC was fine, but in bleeding the system the MC piston seal must have hit a flaw in the bore and tore it. We replaced the MC and all was fine.

IF, you have NO leaks.........your data and conditions leads me to believe your "new/rebuilt" MC is the fault.

[Joe R.]

Re: Brake Bleeding

Ross,

How many bleeders do you have on the REAR calipers? More than one on each caliper I expect.

JR

[Ross F.]

Re: Brake Bleeding

Steve. I'm going to use your method and see if I can isolate the problem to a particular caliper.
Richard it's a 75 roadster and I'm using dot 3. As with most corvettes someone who cared not about originality replaced the MC and booster. So I bought the correct items complete. So rebuilt by others. Always PB. Say so on the firewall. Bought as a set so hopefully the pushrod is ok. No leaks I have a lift and I've looked extensively.
Joe, the fronts have one. The backs have two.
Bled them all ad nauseum.

[Steve G.]

Re: Brake Bleeding

Steve. I'm going to use your method and see if I can isolate the problem to a particular caliper.
Richard it's a 75 roadster and I'm using dot 3. As with most corvettes someone who cared not about originality replaced the MC and booster. So I bought the correct items complete. So rebuilt by others. Always PB. Say so on the firewall. Bought as a set so hopefully the pushrod is ok. No leaks I have a lift and I've looked extensively.
Joe, the fronts have one. The backs have two.
Bled them all ad nauseum.

Ross
Make sure you try the first one, or do a visual check of where the pistons are relative to the pads. It can take a lot of pedal action and fluid to move 16 pistons from the bottoms of their bore to up against the pads where they need to be. This doesn't happen on the original lip type seals because the lip seals offer very little resistance to movements and the springs have pushed them out against the pads before yu evan began bleeding. Every stroke that moves this unopposed movement feels like air. We see this daily when doing brakes and you've pushed the piston all the way down. Never opened the hydraulics. Push the pedal down, no brakes. Even with only two (larger) pistons it takes a few pumps to get the pistons out.

Steve

[Ross F.]

Re: Brake Bleeding

Thanks Steve.
But wouldn't the power bleeder push the Pistons out?
And I've pumped the brakes til I got a leg like Arnold, and nothing has changed.

[Steve G.]

Re: Brake Bleeding

Thanks Steve.
But wouldn't the power bleeder push the Pistons out?
And I've pumped the brakes til I got a leg like Arnold, and nothing has changed.

The power bleeders I have used really only made enough pressure to push through a line with an open bleeder. That's all they were intended to do. If this o-ring conversion uses square cross section rings like on a typical single piston floating caliper, I would say probably not. And remember, you will not be moving the piston when the bleeder is open.

The reason I lean so much to believing that is that your plugging the mc is a good and valid test. I would have done it only slightly different, blocking at the rubber hoses as with the vice grip. This confirms your master cyl is good.

Once you know the mc is good would be extremely unusual to have a problem downstream that is not leaking fluid on the ground. If the mc is working,ie it pumps fluid and holds, where is the fluid going?

Which brings to mind, as you've been working the pedal with all the screws closed after you finished bleeding, is the level of fluid going down at all?
Steve

[Steve G.]

Re: Brake Bleeding

Ross,
Thought of something else you should check if you haven't already. I've never seen it affect a system in the way you're describing, but whenever a master cyl has been changed, and especially when both booster and mc are being replaced it's important to check the length of the pushrod that extends from the booster into the mc.

You take an accurate measurement from the mounting face of the booster to the tip of the pushrod. That length must be slightly less than the measurement from the bottom of the hole in the mc piston that the pushrod sits in to the mounting face of the mc. The important thing here is that when the mc is bolted to the booster the pushrod is not preventing the mc piston from returning to the very end of it's stroke.

The usual symptom associated with this is the brakes dragging after they warm up, but it may be possible that it is preventing the mc from being able to recover fluid from the reservoir.

Steve

[Richard M.]

Re: Brake Bleeding

The power bleeders I have used really only made enough pressure to push through a line with an open bleeder. That's all they were intended to do. If this o-ring conversion uses square cross section rings like on a typical single piston floating caliper, I would say probably not. And remember, you will not be moving the piston when the bleeder is open.

The reason I lean so much to believing that is that your plugging the mc is a good and valid test. I would have done it only slightly different, blocking at the rubber hoses as with the vice grip. This confirms your master cyl is good.

Once you know the mc is good would be extremely unusual to have a problem downstream that is not leaking fluid on the ground. If the mc is working,ie it pumps fluid and holds, where is the fluid going?

Which brings to mind, as you've been working the pedal with all the screws closed after you finished bleeding, is the level of fluid going down at all?
Steve

This is what I was trying to bring up in Post#5. Blocking the ports directly at the MC disallows piston travel emulating the system downstream. When the MC is blocked at the source, and internally bled with fluid, i.e. no air in the bore, then the travel is restricted. When connected to the entire brake system, with the ability to move 16 pistons downstream, the MC piston moves further in the bore.

IF....there is a internal fault in the MC seals, the fluid will bypass between the internal return circuits. The pedal will feel hard momentarily, then will hit the internal leakage point, and go to the floor.

Just because the MC is new, and because it came from a "human" rebuilder, it does not mean it is functionally "correct". I have horror stories in my experiences as I'm sure others do, when we get some rebuilt items from outside sources. Perfection is not always the case.

If this condition exists in his MC, and if there are NO leaks, then the fluid has to go somewhere. Technically it goes nowhere. It stays in the MC and goes right back into the reservoir. This condition may be seen inside the reservoir with the cover removed. A helper pushing on the pedal VERY slowly with the cover off may be helpful while the observer can see the condition and fluid flow moving rapidly inside the MC bowls. Since DOT 3 is in use, (which I HATE with a paint removal passion), then use due caution when peddling with the covers off.

Ross,
Thought of something else you should check if you haven't already. I've never seen it affect a system in the way you're describing, but whenever a master cyl has been changed, and especially when both booster and mc are being replaced it's important to check the length of the pushrod that extends from the booster into the mc.

You take an accurate measurement from the mounting face of the booster to the tip of the pushrod. That length must be slightly less than the measurement from the bottom of the hole in the mc piston that the pushrod sits in to the mounting face of the mc. The important thing here is that when the mc is bolted to the booster the pushrod is not preventing the mc piston from returning to the very end of it's stroke.

The usual symptom associated with this is the brakes dragging after they warm up, but it may be possible that it is preventing the mc from being able to recover fluid from the reservoir.

Steve

Frank Dreano had a similar condition in his Power Brake system in his '63 a few months ago. The "First" rebuilder used the wrong pushrod.

Frank experimented to help diagnose the problem. He spaced the MC to Booster junction with some washers. This brought the MC out forward and allowed the pushrod to be at the proper start point IIRC. Problem confirmed. He sent the 2 units out to a "more knowledgeable" rebuilder, corrected, and it worked flawlessly when it returned. He may expand on this in more detail when he sees this.

----
Rich

[Frank D.]

Re: Brake Bleeding

In my case (remember there are two push rods)...the M/C to booster rod was too long and the pedal to booster rod was too short; resulting in rock hard pedal but touchy brakes....I mean like put you through the windshield touchy. It turned out some internal part of the rebuilt booster was in backwards (sorry I'm not articulate on booster internals) was installed backwards by the "professional" rebuilder. Reversing that (and paying for another rebuild) fixed it. Doesn't sound like exactly the same problem but might be worth some investigation..

[Steve G.]

Re: Brake Bleeding

This is what I was trying to bring up in Post#5. Blocking the ports directly at the MC disallows piston travel emulating the system downstream. When the MC is blocked at the source, and internally bled with fluid, i.e. no air in the bore, then the travel is restricted. When connected to the entire brake system, with the ability to move 16 pistons downstream, the MC piston moves further in the bore.

IF....there is a internal fault in the MC seals, the fluid will bypass between the internal return circuits. The pedal will feel hard momentarily, then will hit the internal leakage point, and go to the floor.

Just because the MC is new, and because it came from a "human" rebuilder, it does not mean it is functionally "correct". I have horror stories in my experiences as I'm sure others do, when we get some rebuilt items from outside sources. Perfection is not always the case.

If this condition exists in his MC, and if there are NO leaks, then the fluid has to go somewhere. Technically it goes nowhere. It stays in the MC and goes right back into the reservoir. This condition may be seen inside the reservoir with the cover removed. A helper pushing on the pedal VERY slowly with the cover off may be helpful while the observer can see the condition and fluid flow moving rapidly inside the MC bowls. Since DOT 3 is in use, (which I HATE with a paint removal passion), then use due caution when peddling with the covers off.




Frank Dreano had a similar condition in his Power Brake system in his '63 a few months ago. The "First" rebuilder used the wrong pushrod.

Frank experimented to help diagnose the problem. He spaced the MC to Booster junction with some washers. This brought the MC out forward and allowed the pushrod to be at the proper start point IIRC. Problem confirmed. He sent the 2 units out to a "more knowledgeable" rebuilder, corrected, and it worked flawlessly when it returned. He may expand on this in more detail when he sees this.

----
Rich

Rich,
I have used this method of diagnosis extensively in my career. If the mc is moving fluid while the bleeder is open and holds pressure when blocked at the top of it's travel the likelihood of anything wrong in the mc is extremely low to non existent. Holding pressure,even at the top of stroke, ensures the piston seals are holding. In order to create pressure anywhere in the stroke the seals have to be clear of the ports. The only thing that changes as the piston seals travels down the bore is the bore itself which is just a straight bore with no ports. Additionally, this is a dual circuit mc. What are the chances of both bores having some kind of catastrophic malformation in the same place in each seals respective travel?

The symptom usually associated with the out of adjustment pushrod is brakes working normally for the first few few applications then starting to drag. The drag increases to the point the brakes are fully applied. They won't release until they cool down then it all starts over. The reason for this is that the pushrod is too long and not allowing the piston to return to it;s proper rest position where the port to the reservoir opens. The fluid is trapped , the reservoir out of the picture. As the brakes warm up the fluid expands. It can't move backwards to the mc, so it applies the brakes, which makes them hotter and the fluid expands more ...

A master cyl can hold properly at the top of it's stroke and still leak internally through it's stroke. But this is not the symptom you get. What happens in these cases, which is the malfunction of the mc in about 99% of the cases, is that you will have brakes but as you hold them on the pedal will gradually creep to the floor as the fluid by-passes the seal. If you stand on the pedal as hard as you can the leakage slows down or even goes away. It is the nature of a lip seal. The higher the pressure the harder the lip is pushed against the cyl. The lighter the application the faster the pedal moves to the floor.

Steve

[Richard M.]

Re: Brake Bleeding

Rich,
I have used this method of diagnosis extensively in my career. If the mc is moving fluid while the bleeder is open and holds pressure when blocked at the top of it's travel the likelihood of anything wrong in the mc is extremely low to non existent. Holding pressure,even at the top of stroke, ensures the piston seals are holding. In order to create pressure anywhere in the stroke the seals have to be clear of the ports. The only thing that changes as the piston seals travels down the bore is the bore itself which is just a straight bore with no ports. Additionally, this is a dual circuit mc. What are the chances of both bores having some kind of catastrophic malformation in the same place in each seals respective travel?

The symptom usually associated with the out of adjustment pushrod is brakes working normally for the first few few applications then starting to drag. The drag increases to the point the brakes are fully applied. They won't release until they cool down then it all starts over. The reason for this is that the pushrod is too long and not allowing the piston to return to it;s proper rest position where the port to the reservoir opens. The fluid is trapped , the reservoir out of the picture. As the brakes warm up the fluid expands. It can't move backwards to the mc, so it applies the brakes, which makes them hotter and the fluid expands more ...

A master cyl can hold properly at the top of it's stroke and still leak internally through it's stroke. But this is not the symptom you get. What happens in these cases, which is the malfunction of the mc in about 99% of the cases, is that you will have brakes but as you hold them on the pedal will gradually creep to the floor as the fluid by-passes the seal. If you stand on the pedal as hard as you can the leakage slows down or even goes away. It is the nature of a lip seal. The higher the pressure the harder the lip is pushed against the cyl. The lighter the application the faster the pedal moves to the floor.

Steve

Steve, You done more brakes than me I'm sure, but I have done a few myself. Seen some strange issues.

What if the lip seals were installed backwards by the supplier, and the rebuilder didn't notice it? Years ago I had a Jaguar dual rebuilt MC and the seals were reversed. I'm not positive, but I recall it may have been a similar condition.

If I had that MC in front of me it I would open it up to verify it's assembled correctly. Or I would install the original one which was removed as a baseline. That or remove the booster from the system to eliminate pushrod issues.

Jack needs to try some of the tests, and then results will tell us what may be failing. I honestly hope it's something simple for him. He's frustrated at this point I'm sure.

Rich