Ok. Here's the deal. When I did my engine swap/front suspension rebuild last summer, the entire brake system was drained of fluid. (Not intentionally. My brake line plugging method didn't work out as I had planned.) Since then, I haven't been able to get a great pedal. It was ok, but not great. The car sat for about 3 months over the summer. Then my brakes felt really bad. I bled the brakes. Still had spongy pedal. I checked the calipers, and the LR seemed to be sucking a little air. I changed both rear calipers, and bled the brakes. It seemed better for about the first 2 stops, then... spongy pedal. I buy a vacuum pump. Vacuum bleed the brakes. Decent for about 5 stops... spongy pedal. I gravity bleed the brakes. Decent for about 5 stops... spongy pedal. Do you see a pattern here? I've found that if I turn the car off. Then restart it and drive it. The pedal is decent for a few stops, then it will barely keep the car from moving in gear!!! What's wrong? Do I need to remove the master cylinder and bench bleed it? Do I need to replace the master cylinder and booster. (I can get a brand new one from a '77 for free, but will it work?)

Please help. I'd really like to have the car going by next weekend! It's the Vette Drag in Chattanooga!

:confused:

TIA,

 

bad master cylinder ??

 

Do you get air out when you bleed it? The crappie seal design and the piston anti-knockback springs are notorous for sucking air past the caliper seals if there is much rotor runout at all. Find the caliper(s) that have air when you bleed and check the rotor runout with a dial indicator. Have the rotors turned if they are out of spec.

 

That's the thing. I don't get any air when I bleed. I took the car out for a little while today. Sometimes it would have decent pedal at first, but then it would keep sinking down to the floorboard under constant pressure. Does that mean it's the master cylinder? It will be Tuesdy before I can get the new master cylinder an booster since I live in Cookeville, TN (Read adzhole of the world!). Auto parts stores here suck! I guess I'll try the booster and m.c., then if that doesn't work, I'll be buying a dial indicator.

:confused:

 

You know that if the system sat for 3 months dry there may have been some corression set in the MC and even the calipers that you did not replace. I would for the nominal cost, pickup a new master cylinder at say Auto Zone, bench bleed it fully, install and rebleed the wheels with all fresh fluid. Then let's see where you stand.

 

I would try the MC first, sounds like you are getting bleed by around the seals on the plunger. Just my .02 worth.

YBNormal...drive a vette
www.angelfire.com/al/swhite7/doug.html

 

Get a vacuum pump with a guage and pump up the booster to about 18-20 psi. It should hold that vacuum. If it leaks down change the booster. I had a similar problem with mine. The booster wasn't holding vacuum and as it gradually leaked down the pedal would get softer and softer.

 

That's the sign of master cylinder piston leakage. If the fluid drained out and exposed the M/C seals to air, then they may have dried out and cracked. replace the m/c and you should be fine...keep an eye on the front calipers to watch and see if they leak.