C3 Soft Brake pedal

[Bob H.]

I hated to even post such an issue but am lost. I have replaced and bled brakes on several Vettes over the years really with no major issues. The 72 I purchased had sit for a long time and although it had a soft pedal, the brakes were good enough to get it home. Fast forward, I replaced all pads, calipers, soft brake lines and flushed the system. I also replace the m/c,(bench bled) and my pedal is not better. It goes nearly to the floor and is mush. Can the proportioning valve cause any issue in bleeding?? Everything else has been changed, I have gravity flowed all 6 points with no issue, got a small amount of air in the beginning but good flow now.

[Joe L.]

Re: C3 Soft Brake pedal

I hated to even post such an issue but am lost. I have replaced and bled brakes on several Vettes over the years really with no major issues. The 72 I purchased had sit for a long time and although it had a soft pedal, the brakes were good enough to get it home. Fast forward, I replaced all pads, calipers, soft brake lines and flushed the system. I also replace the m/c,(bench bled) and my pedal is not better. It goes nearly to the floor and is mush. Can the proportioning valve cause any issue in bleeding?? Everything else has been changed, I have gravity flowed all 6 points with no issue, got a small amount of air in the beginning but good flow now.

Bob------


All I can offer is this: there is no service job on a C3 Corvette that I fear more than bleeding the brakes. None. Period.

[Marc K.]

Re: C3 Soft Brake pedal

Hi Bob,
I completely agree with Joe that bleeding the brakes on a C3 can be a challenge. I've replaced numerous calipers and the master cylinder on my 1970. After replacing the MC (bench bled), I had a soft pedal as well. Even bleeding the calipers over and over, the pedal did not get much better. I decided to make fixtures from brake line stubs and rubber tubing to connect to the outlets of the MC, then empty back into the reservoir, and slowly pumped the brake pedal numerous times. I was working alone, so I don't know if I got any bubbles out, but after bleeding the calipers again, it solved the problem for me. Don't know if this would work for you, but just my two cents. Good luck!
Marc

[Richard M.]

Re: C3 Soft Brake pedal

I feel your pain. In my experiences with all of the C2/C3 bleeding, which I always hated, the soft pedal syndrome has always been air entrapment ..... somewhere.

On every occasion the rear calipers always seem to be the nightmare. They're furthest awaywith 2 bleeders.

I don't like the Motive bleeder. I have a phobia of a leak at the MC and always had fears of blasting fluid all over the car if it leaked under pressure. Never will I use one!

I am a proponent of gravity bleeding. Much time waiting but I don't care. I go do other things while it bleeds itself. I just keep a eye on the MC and keep it topped off.

I remove all 6 bleeders(2 F, 4 R) and wrap Teflon tape on the threads. I use my Mity-Vac to get a fluid flow on each corner. Just to see a little is fine. Close bleeder each corner

I begin the process on LF first. This way any MC or follow on line air gets evacuated from the system immediately. Any air is now going to be beyond the MC and LF plumbing. Then I do the same on the RF. I then connect clear hose to fluid filled jars to each front single bleeder and crack open until I see flow. Wait, Watch, MC top off. Then I close the front bleeders when no more air is seen.

Then to the rears. I connect 2 short clear hoses, to a plastic Tee, then the Tee output to a single long clear hose to a fluid filled jar. I do this on each side.

Let gravity work at both rears. Wait and watch flow and MC levels. Give the calipers a tap with a rubber mallet often to agitate and release any air in each caliper half.

Wait. Watch. Top off MC. When no more air is seen I close all bleeders.

Always finishes up with a hard pedal.

Notes.

If new MC, bench bleed it.

If old MC, keep in mind gravity bleeding will save it. If you try to pedal bleed a old one, when the inner seals hit the area beyond normal previous piston travel, the seals could be torn by any corrosion in the bore, particularly if Dot3.

Rich

[Edward J.]

Re: C3 Soft Brake pedal

To add to Rich’s post I have had luck bleeding the fronts first since the are the closest to the master cyl. And once you gravity bleed you find it’s the only way to do this. Make sure the top in off on master also keep it topped off, It’s a waiting game as Rich says. I forgot when bleeding rears bleed the outsides first, you may have to do each caliper a few times after a few pumps of the pedal

[Patrick H.]

Re: C3 Soft Brake pedal

I don't like the Motive bleeder. I have a phobia of a leak at the MC and always had fears of blasting fluid all over the car if it leaked under pressure. Never will I use one!

Rich

Buy the metal top that Motive offers and use a C-clamp. Avoid the plastic top and the chains.
Zero issues and you don't spill a drop.

I'll be using mine later today on a 65.

[Bob H.]

Re: C3 Soft Brake pedal

still battling soft pedal but the front seem to be working ok. The backs are the problem. I have run a ton of fluid through them. You can see them clamp down but I guess there is not good pressure. Can the proportioning valve cause this problem? It's he only thing in the system not new.

[Joe L.]

Re: C3 Soft Brake pedal

still battling soft pedal but the front seem to be working ok. The backs are the problem. I have run a ton of fluid through them. You can see them clamp down but I guess there is not good pressure. Can the proportioning valve cause this problem? It's he only thing in the system not new.

Bob------


There is no proportioning valve in a standard or J-55 1967-77 brake system. What folks describe as a "proportioning valve" is merely a brake fluid distribution block and pressure differential warning switch. However, if the switch is off-center, I believe that could cause a bleeding problem. It's easy to tell if this has occurred, though---the dash brake warning light will be on with the parking brake off.

[Bob H.]

Re: C3 Soft Brake pedal

Thanks Joe. I don't have a light with the manual brake off.I don't trust that with this car so maybe I could put a meter on the switch to act as a backup test. I assume it goes to ground if not equal.

[Richard M.]

Re: C3 Soft Brake pedal

Thanks Joe. I don't have a light with the manual brake off.I don't trust that with this car so maybe I could put a meter on the switch to act as a backup test. I assume it goes to ground if not equal.

Correct. It grounds the "BRAKE" lamp circuit with Ign Key ON.

The PDWA switch won't cause a soft pedal unless it's leaking.

When bleeding the rears did you Tee in both bleederss and gravity bleed?

The only other thought is possibly your MC has a internal issue allowing a leak from the rear bore to leak past the seal to the front. It may be worthwhile to open it and inspect the internals.

If this is a power brake system the piston rod length is critical too. Not sure if that applies here.

Rich

[Bob H.]

Re: C3 Soft Brake pedal

I did gravity bleed a LOT! And they actually flow better then any I have done before. It has manual brakes and has the deep piston. The m/c is new but I think I am going to look at it again. Frankly, I may have ignored that when I bench bled, I could see great flow on the front but it seemed to be less on the rear. Maybe I should have looked closer at that. I guess being new, I blew it off.

[Richard M.]

Re: C3 Soft Brake pedal

I did gravity bleed a LOT! And they actually flow better then any I have done before. It has manual brakes and has the deep piston. The m/c is new but I think I am going to look at it again. Frankly, I may have ignored that when I bench bled, I could see great flow on the front but it seemed to be less on the rear. Maybe I should have looked closer at that. I guess being new, I blew it off.

Bob,

To me that could be a internal fault... Maybe a nick in the seal or even a porosity in the bore.

Curious.... Which new MC is it and where did you get it? I've had nightmares with some repro castings. Lately I've done the waiting game getting originals sleeved. Painful to wait but when done its a good feeling.

Lastly, when bleeding rear caliperss, sometimes air gets trapped inside. I use a rubber mallet and smack the caliper halves to jog it up and out. Your bleederss are up top, correct?

Rich
PS, As Joe said, Argh when bleeding these. Patrick's Motive use is sounding more and more pleasing if all else checks out. I'd probably get converted to that since I'm a Dot 5 guy.

[Patrick H.]

Re: C3 Soft Brake pedal

Motive worked slick as can be on the 65 last Wednesday.
Firm pedal, stops on a dime.
Not a drop spilled from the MC.

[William F.]

Re: C3 Soft Brake pedal

Please tell us exactly how you use Motive bleeder and how you keep MC from spilling when you remove adaptor from MC.
Thanks

[Mark E.]

Re: C3 Soft Brake pedal

Motive worked slick as can be on the 65 last Wednesday.
Firm pedal, stops on a dime.
Not a drop spilled from the MC.

I've used the Motive pressure bleeder for years. Quick and easy with good results. Using a C-clamp is the way to go. Also, inspite what the directions say, do not put any fluid in the bleeder. Air only.