78- Constant air in brakes

[CorvetteMike]

I have a 78 C3 had the brake lines changed last year and brakes worked fine. Then randomly had spongy brakes so I replaced the master cylinder bench bleed it properly and bleed the whole system afterwards. Had no issues for 20 Kmart form brakes then went spongy again. (No leaks anywhere ether). Would like any advice on how to tackle this issue.

 

[Roughrider]

Not sure but is there a special bleeding sequence or could the master be bleeding fluid into the booster? But I know you said no leaks?
Do you have anti lock brakes? I suppose there could be a problem in there.

 

[CorvetteMike]

Not sure but is there a special bleeding sequence or could the master be bleeding fluid into the booster? But I know you said no leaks?
Do you have anti lock brakes? I suppose there could be a problem in there.

No special sequence (what was recommend which was furthest from master cylinder to closest) and no antilock system.

 

[Roughrider]

Hmmm what about a proportioning valve?
If I remember properly they are also involved in a bleeding process?

 

[CorvetteMike]

Yes it had one of those I didn’t know that needed to be bleed and if so how do I?

 

[Roughrider]

I’m not sure but see if you can do a google on it.

 

[Tripod]

Are you aware that there are inner and outer bleeder screws on the rear calipers?

Left rear inner
left rear outer
right rear inner
right rear outer
right front
left front

You can go:
Right Inner
Right Outer
Left inner
Left outer
Right front
Left front

Start with the inners on the rear is the point.

 

[CorvetteMike]

Are you aware that there are inner and outer bleeder screws on the rear calipers?

Left rear inner
left rear outer
right rear inner
right rear outer
right front
left front

You can go:
Right Inner
Right Outer
Left inner
Left outer
Right front
Left front

Start with the inners on the rear is the point.

Yes I did. Thank you for the tip though. I’m going to start with the proportioning valve and re bleed the calipers and give an update reply.

 

[GSBob]

its a fairly common problem with C3s. Assuming they must be sleeve calipers by now it’s more than likely the lip seals. I’ve change a lot of them over time from lip seals to o-rings. The kits aren’t too much money. Usually under 80 bucks for either fronts or backs. When hot they tend to suck in air and don’t leak. You bleed them and within a short time they get soft again.
just a thought since you’ve tried everything else with no cure.

 

[Roughrider]

Any update on your brake problem?

 

[CorvetteMike]

So I’ve been away fighting wildfires so haven’t had time to do it. But just purchased a rebuild kit for both back calipers. Last time I bled the brakes there was only air bubbles coming out of the one back left caliper and it formed up again.

 

[MuskokaMac]

This is a really common problem on c3's & c2's with rear disk brakes. When the cars were orignally manufactured the rear hub & rotor were machined together, after the years go by and the rotors are pulled off & then not put back in there original location, or maybe replaced you end up with rotor run out. The reason this effects these cars is because the original style seals were designed to keep brake fluid in, they were never designed to keep air from getting in because when the rotor runs true the problem doesn't happen. The brake pads on a c3 caliper ride on the surface of the rotor all the time & if you have rotor run out it will cause the pads to move in & out which in turn sucks air into the rotor, air that probably contains moisture which in turn rusts out the sleaves in the caliper. The more run out, the quicker the problem happens. You'll need a dial indicator and a shim pack that you can purchase at Nappa, Rock auto, etc. there are numerous articals on the net that will walk you through the process.

 

[Brian Keery]

There's your answer....