LoneStar26047

I need help diagnosing a brake issue. I bled the breaks...each wheel three times...following the correct steps. The pedal came up nicely. Everything appeared to be correct. Drove the car for over half an hour with no problems and no spongy breaks. Parked the car for 48 hours and the pedal almost went to the floor. Bled breaks again. Again the pedal comes up nicely. 24 hours later...spongy pedal.

There is no indication of a leak anywhere. And driving the car or letting it sit seems to have no bearing on the pedal going spongy so I don't think I have a runout issue. (I also made sure the rear rotors went back on the correct studs.)

I can bleed the brakes, bring the pedal back up to pressure and step firmly and evenly on the pedal and I feel it gradually travel to the floor....but no indication of a leak anywhere! The level in the MC never goes down...unless I am bleeding the breaks. That is the only time I get any change in the fluid level.

Is it possible that my MC went bad while I was working on the rear calipers? If I missed some air in a line would this cause this problem? I really don't want to replace the MC if I am just not getting all the air out of the line or if I have some other issue.

dannyman

If you are not losing fluid in the MC and have no apparent leaks, then I was suspect you still have air in the system. Bleed, bleed and re-bleed.

mrvette

Rebuild your Stainless lined calipers with O ring pistons/rings, and toss the backing spring away, then replace the power breake booster with a Hydroboost unit....and kiss your brake problems away....

LoneStar26047

Spent way too much time this weekend working on something that should not have taken so long! At least I think I have eliminated every possibilty but a bad Master Cyliner. I gravity bled the brakes for a couple of hours on Saturday, tapping on everything with a rubber hammer as I was doing it, but they never really came up to acceptable pressure on the pedal. So I gave up on that and bought a power bleeder from Harbor Freight. Pulled about 1 1/2" cups of fluid thru each of the four lines. Brake pedal never did get firm. So now I am thinking there is no way I have any air in any lines. Still no sign of a leak anywhere and the fluid level never drops unless I am bleeding. So next step is a new MC! Woo Who!

Durango_Boy

You can rebuild the one you have too.

PETKAH

Had the exact same problem with my '75 last month. Put on a new MC and that worked for a week. Replaced calipers on back only and that solved the problem. No visable leaks but air was getting into the calipers somehow.

Durango_Boy

Probably rotor run out, where the rotor wobbles and turns the caliper into an air pump, sucking air in. This is common, but the OP said his pedal pressure would bleed off without the car being moved. Run out will only be a problem if you are driving.

LoneStar26047

I didn't want to hear this.

I did break the seal on the rear calipers by seperating the two halves, (but never touched the piston seals). There is only a small o-ring connecting the two halves, so not many places for an air leak to develop and since you can buy rebuild kits for calipers I figured not much could go wrong with reconnecting the two halves. I replaced the o-ring when I put the two halves back together and tightened the calipers with a break-over bar. I just don't think I have a caliper leak.

DB is correct. The spongy pedal happens even if I am not driving so I think I can eliminate runout. Also, the breaks are getting worse every time I bleed them (they never come up to a firm pedal anymore), which I think would mean the more I mess with pumping the system the more I deteriorate the seals in the MC. Once I replace the MC I guess I will know in a few days if I need to replace the rear calipers.

Dr.G

I had the exact same problem. I replaced a caliper, replaced both front rotors and pads ( runout is now less than.003). But, I think the master cylinder was the biggest part of the problem. Bought a rebuilt one from a local auto parts store, problem solved. If you sit in the car with the engine off and step on the brakes and can feel the pedal slowly going to the floor, you have a bad master cylinder,( from the Chiltons manual on corvette repair)
Dave

LoneStar26047

That is exactly what I am feeling when the engine is off! A sinking feeling on the pedal ... and in my chest! Actually, if I can stop the bleeding (pun intended!) by changing out the MC I will be soooo happy!

Thanks so much to everyone. Your input sure beats .

bonehead2

You master cylinder is by passing internally, time for a new one.

kevinator80

Don't waste anymore of your time. Change or rebuild the mc and be done.