Help! Brake Problems:

[401KVet]

I just had a new master cylinder and new power booster installed which I purchased from Mid America. The shop that did the insullation is suppose to be very good.
First off they had a problem bleeding and getting a good peddal with the insulation. Now, when I drive it, sometimes the pedal gets really hard and I virtually have no brakes. Pump them a little, then all good until the next time. Not to often does it do this. If I sit in the car before starting it, and pump the brakes, the pedal will get hard. I start the car, the brake light is on, pump the brakes, it goes off.
The calibers were rebuilt about 3 years ago. I'm taking it back to the shop in the morning, but I'm beginning to think they don't have a clue. Pads are good.
What can I tell these guys to look for ;help

 

[GTR1999]

Sounds like air in the system and a leak if the light came on.
Forget the shop if you have to keep paying for incorrect work. Buy a Motive Bleeder, pump it up to 12 psi and watch the gauge. If it drops you have a leak. Since you bleed each circuit you will know where to look. Could be a weeping caliper or connection. Fix it and bleed the system. Then check the rotor runout in each rotor, they must be under 005 total.Check the bearing endplay- should be 0015-002 ideally but you get by with up to 003,maybe 004" depending on rotor runout.
If these are not checked and ruled out you will be throwing money and time at it or paying someone to do the same.
Good luck

 

[401KVet]

I dropped my car off back again at the same shop today. They are trying to figure it out.
Even though they may know what they are doing, I will forward this info to them.

Thank You,

 

[petes74ttop]

Brake Problems!

#1: Pull the wheels off, remove the pads and confirm that there are no leaking calipers.
#2: Then check that the new M/C is good remove the two lines and install plugs. The brake pedal should now be "Rock Hard".
#3: A BAD power booster will require a LOT of pedal pressure to stop your Vette. I had two bad in a row, replacing one requires a visit to the Chiropractor, doing it 3 times, forgetaboutit!
#4: You have a 30 year old "Proportioning Valve" (actually a brake distribution block and switch). It may need to be re-centered or the "O" rings may be shot and the unit need to be replaced.
#5: As mentioned you may have excessive run out but that usuall happends if you have good brakes for a while and then after so many miles then the brakes fail and after you bleed them the problem comes back.

 

[Vettehead Mikey]

1980 Corvettes actually have a proportioning function incorporated into the valve. I think '78 was the first year for that.