Brake Bleeding

[Joe L.]

Re: Brake Bleeding

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.







Rich

Rich------


Yes, this is a common problem with commercially rebuilt (i.e auto parts store) master cylinders, especially the low cost variety. If you can buy a rebuilt master cylinder for 20-40 bucks, or so, don't think of that as a good deal. Think of it as a prescription for frustration and more work. Why? Well, master cylinders in this price range (and maybe some in higher price range, too) will virtually never be rebuilt with a new bore (i.e brass or stainless steel). Instead, they'll almost always have the bore heavily honed to "clean up" corrosion damage. These just don't hold the pressure required. So, the installer figures that since he has a "new" master cylinder, the problem must be somewhere else. Thus begins the fruitless quest for another problem.

[Steve G.]

Re: Brake Bleeding

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

Rich,
If the seals are installed backwards it won't hold at the top of it's stroke either, which is why I say that his was a valid test of the mc. And you can test further if you think think that as soon as you leave top of the stroke the issue appears.

Block the lines as previously done. The pedal is now solid and right at the top. While someone maintains a moderate pressure on the pedal someone else cracks open the line at the mc slightly. As the fluid leaks out, the pedal drops, Retighten the line anywhere you like through the pedals travel and confirm that it will still hold in that position.

Steve

[Steve G.]

Re: Brake Bleeding

The more I think about this the more I think this is the pushrod issue. If the piston is not returning to it's proper position not only can it not allow the fluid to escape the bore, it can not "reload" the bore. In a situation where all you replaced was the mc you won't need a lot of fluid to set the pistons, so a single stroke after power bleeding will yield brakes. But in this case it needs to "reload" the bore to push all those 16 pistons out.

If he were manually bleeding he would have noticed this in the bleeding process. He wouldn't get more than the first stroke of fluid, and the fun would have begun there.

Check the pushrod adjustment b4 going on to anything else.
Steve

[Domenic T.]

Re: Brake Bleeding

Rich& Steve,
What about the compensating valve? Position might put the internals so the fluid returns to the MC.

DOM

[Steve G.]

Re: Brake Bleeding

Rich& Steve,
What about the compensating valve? Position might put the internals so the fluid returns to the MC.

DOM

Proportioning valves are all internal and there is no return line to the mc. They are a pressure regulator. Degree of restriction, failure is either open or closed. If open, fluid gets by without restriction and symptom is rear brakes lock up before the frt. If malfunction is closed, no fluid makes it to the rear. Pedal is high and solid with no rear brakes. Diagnosed at bleeding, can't get fluid to rear wheels. Any other malfunction has to be fluid leaking to the ground. I don't think these use a proportioning valve.

Other is pressure differential switch valve. All that does is turn the light on if pressure drops on one side and spool shuttle over and trips the switch. Only malfunction is spool seizes and won't move, light doesn't come on when there is a pressure drop on one side. Leakage results in fluid drip to the ground.

Steve

[Domenic T.]

Re: Brake Bleeding

Steve,
I was talking about the compensating valve, the one that squirts fluid under pressure back into the MC thru that tiny apx 1/16 hole. When I twisted a wrench for a Chevy dealer I remember similar problems that involved the return to MC valve as the book called it then. The most common failure I remember was brake lockup because the rod was not adjusted on the older cars. By the the lockbox were all there in the shop after a mechanic rebuilt & adjusted the rod. You may remember the adjustable brake rods the older cars had.

Dom

[Steve G.]

Re: Brake Bleeding

Steve,
I was talking about the compensating valve, the one that squirts fluid under pressure back into the MC thru that tiny apx 1/16 hole. When I twisted a wrench for a Chevy dealer I remember similar problems that involved the return to MC valve as the book called it then. The most common failure I remember was brake lockup because the rod was not adjusted on the older cars. By the the lockbox were all there in the shop after a mechanic rebuilt & adjusted the rod. You may remember the adjustable brake rods the older cars had.

Dom

Sorry Dom, I wasn't thinking of that as a valve, rather a port. You're right, it is a valve and yes, I agree, something is blocking one or both of those ports between the reservoir and the bore. With a new mc and booster I think the most probable blockage is the piston not returning all the way, but if that isn't the issue I would certainly be looking at those holes.

Wanted to add something else to my little description of the valves above for the benefit of future diagnosis of other problems.

When the press diff switch sticks and binds it will quite often move over and put the light on. Can be close to 0 psi on one end of the spool and as high as 2000 psi on the other end. Where the malfunction shows up is when you have corrected the issue and now the light won't go off. Most of the valve/switches were self centering. If it was sticking it wouldn't centre. So what you did was crack the line on the side that hadn't experienced the failure and increase the pedal pressure until the spool hit the centre and the light went out. Take you foot off and tighten the line. If it flashed off then came on you went past centre and now have to close that side and open the other.

Not all switches were self centering. In those cases some switches required you to do the above, but some required you to undo the nut that held the stem and contact in place and pull it out of the drop in the spool, let the spool centre then re-install the switch stem.

Didn't want the incomplete information in my previous post to cause a mis diagnosis.

Steve

[Steve G.]

Re: Brake Bleeding

Sorry Dom, again missed addressing what you were saying in the first post about the compensating valve. I was answering that yes, the problem is the valve not opening, you were suggesting it wasn't closing and the fluid was simply returning to reservoir.

It is a valve and functions as one but it's design is simply a hole drilled between the res and the bore. When the piston is at the start of it's stroke the reservoir is open to the caliper side of the piston. Once the piston passes the hole, it's closed. The only way back would be through the piston.

His test of the mc by clocking confirms that.

What I'm thinking is that the ports are not opening properly because the piston isn't returning fully. In order to "adjust" the brakes out you have to move a fair volume of fluid. Each stroke should pump a fair bit down and the res level should drop. I'm thinking that with the two ports not fully opened the cyl is not "reloading" properly and not moving much fluid with each stroke.
Steve

[Richard M.]

Re: Brake Bleeding

Steve, et al....Just a FYI.


The Pressure Differential Warning Switch on these '67's is not self centering. You must manually crack a bleeder on the opposite of the "triggered" side to get the piston back at center, and slowly apply pedal while observing the light going off, then close the bleeder.

I'd wager that most of these switches on these cars are not functional, with the internal dual ended piston seized where the o-rings sit. In order to clear and free the piston, one must carefully move the piston in either direction. If the o-rings are visible, they must carefully be "tucked" back int the bore.

A few pics for reference.

Center shaft of double ended piston seen in the contactor opening. Picture a rod with 2 pistons on the ends. When the rod moves to one side, it contacts the contactor and completes the circuit to ground to light the lamp, whose other contact is tied to the IGN 'on' path.
PC230004.jpg PC250008.jpg

Here I'm freeing the ends pushing the piston back and forth with a small drift while lubed with brake fluid after a flush with Denatured Alcohol. I used DOT 5 in this rebuilt brake system. The MC hard lines mount to the ends, where the piston is accessible as well.
PC250009.jpg PC260013.jpg


This was after assembly ready for installation.
pc260026.jpg PC260027.jpg PC260022.jpg
I have another here on a '67 restoration in progress. I'll take better photos of the piston assembly on that one.

While I'm at it here are some photos of the (reproduction) '67 MC showing the ports in question.

PC260016.jpgPC260017.jpgPC260018.jpgPC260019.jpgPC260020.jpgPC260021.jpg

edit......BTW, Only the J56 HD brake system included a Proportioning valve to vary and balance F to R brakes. This in addition to the PD switch.

Rich
p.s. We all have to meet over Jack's garage and figure this out.

[Domenic T.]

Re: Brake Bleeding

Well,
Great minds. I have the SS conversion over to O-rings and now know what to expect. Almost bled them for the first time since restoration last week but my local auto parts said he couldn't get DOT 5. I'm sure it's out there but they mentioned DOT 5 5?
Must be another CA rule! If you can't use it as a eye wash, drink it, or cook with it they take it off the market.
Rich, always nice pics. Steve, now I have a heads up on the O-rings. I have my bleeder set at 18PSI so I don't compress the air and bleed from the caliper up like we do with aircraft O-ring brakes which are about 99% of what is used today. Also, your Pop cycle trick! I found a tool that safely squeezed most small lines. The only problem was that I really found one only!

Dom

D

[Richard M.]

Re: Brake Bleeding

Well,
Great minds. I have the SS conversion over to O-rings and now know what to expect. Almost bled them for the first time since restoration last week but my local auto parts said he couldn't get DOT 5. I'm sure it's out there but they mentioned DOT 5 5?
Must be another CA rule! If you can't use it as a eye wash, drink it, or cook with it they take it off the market.
Rich, always nice pics. Steve, now I have a heads up on the O-rings. I have my bleeder set at 18PSI so I don't compress the air and bleed from the caliper up like we do with aircraft O-ring brakes which are about 99% of what is used today. Also, your Pop cycle trick! I found a tool that safely squeezed most small lines. The only problem was that I really found one only!

Dom

D

Dom, He probably meant DOT 5.1, which is NOT silicone fluid. It's Glycol based, like DOT 3, DOT 4, just a higher boiling point.

Why in the name of gosh did they EVER use DOT 5.1 spec for a glycol based fluid, when DOT 5 is silicone, is totally insane IMO.

Why not DOT 4.1 ?????????????

----

[Frank D.]

Re: Brake Bleeding

As mentioned way back; if the feeling is that the M/C pushrod is too long it takes about 3 minutes to put a couple of 1/8" spacers under the mounting bolts at the power booster and then try things out again. That's one of the ways I found my problem.