75 coupe. Ok guys, please help. I have replaced basically everything. All the lines and hoses, new slotted rotors, new calipers with metallic pads, new booster/MC combo and have completely bled the system at a brake shop. There cannot be any air in the lines. It does mechanically stop but nothing like it did with the old stuff. The brakes will not lock em up or through you in the seat like before. Pulling 13in. of vacuumn.
Is it the booster rod to mc adjustment or do I now need an electric vacuumn pump?

 

Either the brake pads are not bedded to the rotors or the booster is defective. The booster rod adjustment only changes the free play, does nothing to improve braking effectiveness.

 

Pressure bleed thats the only way to do it. a little air always remains trapped if you just vac bleed or even the old pump & bleed method.
Ask me how I know I went thru 2 gallons of fluid before figuring this out

 

It was pressure bled throughly at a brake shop.

 

If brakes will not lock up then your MC is not putting out enough pressure.
Does pedal go to the floor? Mash the pedal(while parked ) and see if it slowly mushes to the floor. & I mean stand on it HARD

 

First of all, these Corvette 4 wheel disc brakes are very good, very reliable, and simple to work on. Just because it is a Corvette, doesn't mean it is complicated. I have gravity bleed all the Corvettes I have owned with no problems. Air in the system would make it feel like a soft or spongy pedal, and from the description, I don't think you have that. If the pedal is soft or spongy, then it is possible air is reentering the system somehow. I agree with PMULLALY, sounds like not enough MC pressure to the brakes.

Get a partner to help you. Jack the car up and have your partner apply the brakes. Now, go around and make sure all four wheels are locked. It is possible you are only getting braking to either the front or rear set. If one or the other set is not locked, then this gets us back to the MC. As I recall, the booster rod to MC adjustment is very important. If not adjusted correctly, it will not let the piston(s) in the MC move far enough to uncover or cover certain small holes in the MC bore. If this happens, braking is reduced or nonexistant on either the front or rear sets, or possibly both. It has been a long time, so I am going by memory here. As I recall, there is a specific measurement and procedure for the rod adjustment. Again, I am kind of winging this since it has been some time since I have done this, but these are things you can check. I hope this helps.

 

13 inches is more than enough. I have run with 11 and it worked fine.

It almost seems like either the front or rear isn't working at all.
I wonder if the prrportioning valve is stuck???

 

If the proportioning valve is bad would it still pass fluid to and out of the calipers like normal. I'm gettin plenty of fluid to all bleeder valves.

 

If your proportioning valve gets stuck it bypasses all the pressure to the front or rear. Now with that said, If you bleed the brakes w/ the old pump & dump method you have to let the bleeder screw off slowly. if you let that pedal fall to quick it will make the proportioning valve activate. GOOD LUCK sometimes you just have to drive it for a while and it sometimes comes back on its own.

 

What type of pads are you running? If you've gone to a competition type pad, the pads are probably not getting warm enough to work under normal driving conditions.

 

Suppost to be hi perf semi metallic.

 

Are they broke in yet? If your slamming the brakes on that hard to get it to try to slide A tire (which my right front does all the time im a hard braker )Maybe they have A glaze on them already.My Reg brake shoes took two or three rides to come in .I hope you have not Overheated and messed up the pads .All the stuff you done, that car should stand straight up when you hit the brakes .I would pull my pads and take a look it should tell you if one or the other(ft or Bk) has been working to hard.