I just put a master cylinder on my '81. I bled about three quarts of fluid through the system without obtaining a hard brake pedal. The warning light was out for about two weeks. It came back on so I decided to do some checking.

I tested the booster by holding a vacuum on it with a Mityvac. It held a vacuum.

I placed a plug in the front port of the mc. Testing the rear brakes I still had a soft pedal. I placed a plug in the rear port and hooked up the front line to the mc. I still had a soft pedal. Finally I plugged both ports on the mc and got a hard pedal.

From this I gather that my mc and booster are good. It seems that I still have some air in the front and rear brake system. Air in the system seems to be the culprit.

Do you guys agree with with my methods and the result?

Craig

 

Try this to see if there is still air the MC.

https://www.corvetteforum.com/forums...-cylinder.html.

You will be surprized where the air hides.

 

Did you bench bleed the MC before installing it? I did on my 68, but it still had some air in it. It’s possible to bleed it while on the car and that seemed to help. I finally got a hard pedal after using a Motive power bleeder.

 

I actually have a comment on your thread. Thanks for posting.

I tilted the master cylinder forward, backward, and even during the bench bleeding process. I'm almost certain it has no air in it.

Haven't tried the level approach. I'm about to try it as I've got to rebleed this darn thing again.

 

Yes I did bench bleed the mc quite thoroughly I thought.

I'm using the pressure method to bleed.

I use the Motive bleeder also. Although I tried the suction method with an air powered bleeder without any success.

 

Three possibilities:

1)The brake like distribution block has a shuttle valve in it. If that valve has shifted to one side or the other (because of a system leak in front or rear brakes), you need to RESET this valve back to center position. The GM book has a procedure for this; but I do it differently. When brake bleeding has been finalized, STOMP on the brake pedal as quickly and forcefully as you can. This will reset the valve to center position in MOST cases. Then, retry the pedal to see if it firmed up. NOTE: This should be done before the brakes are bled.

2) the design of the C3 brake calipers has a little 'air trap' inside each caliper; the GM books tell you to rap on them with a rubber mallet to dislodge it while bleeding the brakes. When I finish bleeding the brakes, I take the car out for a drive and make sure I go over some RR tracks, etc. Then I bleed them a final time.

3) the rubber 'jumper' hoses are old and NEED to be replaced. When new, those hoses are stiff/firm and will not swell from application of pressure. When old, they CAN swell and give you 'spongy' brakes.

 

Yup, with both MC ports plugged and the pedal is firm, everything is A-Ok in the MC.
Thats a great test and confirms the piston and seal are good in the MC.

So, where is the air pocket beyond the MC? I assume you are doing all 6 bleeder screws in order.
If you do not want to purchase a Motive then perhaps a Vacuum tool off EBay will help you. Using the pedal method to bleed usually gets people in trouble because of the Prop Valve, like 7T1 said.

 

Thanks for the detailed post.

1. I use a tool to center the combination valve prior to bleeding. It replaces the switch in the valve. The possibility exists that the valve wasn't centered I guess? I doubt the tool would have screwed in to the valve.

2. I did tap all of the calipers with a hammer during the bleeding process. I even rotated the calipers to a vertical position when bleeding.

3. I have replaced the two front hoses with braided brake hoses. The rear are stainless hard lines.

 

Huh? " I even rotated the calipers vertical" ??? How do you do that on the car, if installed?
The bleeder screws are ALWAYS vertical if installed in the correct place. No caliper can be swapped with another. They are "corner specific".

And the rear hoses were never replaced? $12 ea.

 

Yes I placed the calipers vertical with a board between the caliper pistons as I bled each one. The bleeder screws sit at about two o'clock when the calipers are installed. I figured this was the best way to get all of the air out.

I can't recall if I ever replaced the rear hoses. I ordered a new set today just in case.

 

Based on your posts, I'd bet the rear lines (there must be some flexible lines at each wheel to allow for wheel movement) are your problem. Never tried sticking a board in place of the rotor, but I guess it would work. You would have to crack a bleeder screw to keep the caliper pistons from extending when you removed the board...and that could allow air back into the system. I understand the 'logic' of using the board, but not sure if it was a benefit to you or not.

Hope the rear hoses fix your problem. Even if it doesn't, changing them could prevent a BIG problem down the road.

 

You might want to try a reverse bleed (check out the stuff Phoenix Systems makes) - some people swear by their stuff - others - not so much.

Ok - this is a low probability - but I'll put it out there anyway...

A spongy pedal is difficult to distinguish from a situation where one (or more) calipers are not well aligned (not parallel) to the rotor - this creates a situation where one side of the pad has to move further than "normal" to do it's job - thus giving a low pedal feel, It could also be a buggered caliper seal...

What I'd suggest - is exactly what you already did - but now that you know the Master cylinder is good - move the plugs to the caliper side of the brake hoses... If possible - get a knowledgeable "helper" to very slowly depress the brake pedal as you tighten up the plugs to keep air from getting into the line. First block off the rear lines - then see how the pedal feels - then try blocking off the front lines - then when you have it narrowed down to one axle - one caliper blocked - the other open - you should have the problem narrowed down pretty fast. Once you know which caliper to look at - you're most of the way there.... Carefully measure clearance between top of caliper inner and rotor and bottom of caliper inner and rotor - it should be within a couple of thousanths. If it's parallel - I'd pull the caliper in question and "rebuild it". (New seals and a thorough cleaning.)


GOOD LUCK !!!

 

Interesting perspectives on things Purple92. I like your ideas.

When I get the new rear hoses installed I'll re-bleed everything again. Then re-evaluate the situation from there.

I may as well replace the combination valve while I'm there. I have a new one. Those internal seals can't last forever.

Keep the ideas coming. Stay tuned for updates.

 

It looks like the rear hoses have been replaced with SS braided lines. So no need to change the rear hoses out.

Got the combination valve replaced today. Started bleeding the rear brakes. I will finish the front tomorrow. Chasing the elusive firm brake pedal.

 

I bled the front calipers this morning.

The Wilwood calipers have to be tilted vertically in order to get all of the air out. I tilted and bled them in that position. I had quite a bit of air in the fronts. I'm sure it was because of the combination valve being changed. I decided to make another round of bleeding as I still wasn't happy with the feel of the pedal. This time I used the pedal method with speed bleeder valves. The pedal got better with the completion of each wheel. I just put the wheels back on the ground. So I haven't test driven the car yet. I'm crossing my fingers that this pedal stays somewhat hard.

Headed out...

 

I didn't even get out of the driveway. I could tell that the news wasn't good.

PEDAL TO THE FLOOR!

What in the world am I doing wrong?

 

It sounds like you have power brakes?
Do you have a power brake master cylinder? That would be 1.25" dia and not the 1" dia manual master cylinder.

 

Yes the car is an '81 with power brakes.

The master cylinder is the 11/8" bore aftermarket Raybestos.

 

1.125" my mistake, but it sounds like the wrong or failed master cylinder.

 

I'm thinking it has to be the mc also.

I just ordered another but this one is a Delco.