Thread: Power brake problem

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My '34 coupe has a Corvette power MC mounted under the car (typical for a street rod) and works fine under most circumstances. However, in a panic stop situation, it doesn't work very well. I have to use a lot of pressure on the brake pedal for it to stop, almost like

I don't have any power assist. The rest of the system consists of Granada disc with GM calipers up front and Corvette in the rear. A 10 PSI residual

valve in line for the rear brakes (which what was recommended) but not for the fronts. Everything is new with only about 1000 miles on it. It has done this

since the car was built. The other day I almost hit someone in a panic stop so I would like to get this fixed ASAP. What could cause this?

Any idea what your front and rear brake pressures are???? You say Corvette in the rear.....disc or drum.... ???? What size booster and mastercylinder bore??? What sort of distribution valve are you running....????? Need a bit more information...

Thanks guys. Corvette rear is disc. I will have to dig up the info on the MC and other related parts tonight when I get home.

How much vacuum do you have . You may need a vac. pump to help with engine vac.

Do you have a proportioning valve? I usually run a 2psi residual valve for disc, 10 lb for drum, if your calipers are higher than the M/C, you should use them

Mark

Okay, here is what I have. At Idle I have 19" of vacuum. The front rotors are Granada 11" with small GM calipers (78-83 Monte Carlo) Brake pads are Wagner NA7070AR. All the front brake hardware came from TSM (The Streetrod Mfg. Co.) There kit #1252.
The Rear Brakes are 1974 Corvette. Calipers and Rotors from Stainless Steel Brake. Power brake MC and Booster from ECI kit #EC-510. Residual valve 2 PSI both front and rear. Booster is a 7" and I believe the MC is Corvette that come in the kit. Proportioning valve (distribution block) is also from ECI.
I talked with ECI today and they said it sounds like the rubber hose might be collapsing that connects to the booster. I changed it out today and no change. Now they say my front brakes and MC are wrong and want to sell me a new set up for $525. They say my front calipers are to small. I need the larger GM calipers and new brackets and new GM rotors. And a new MC to go along with it. What the hell?

That sucks, but usually the m/c and calipers need to match volumes. The smaller the m/c. the less effort but more travel, and vice-versa. If they sold it all to you as a kit, they should do something for you.

Mark

The front rotors on most systems are larger then the rear rotors, mainly because the front does a larger percentage of the stopping..... If the rear rotors are the same size or larger that would account for your problem... Might want to try a proportioning valve in the rear and see if that helps..... You probably wil need to go to the larger front rotors, but ECI should certainly have known this if they're in the business of selling brake systems...

Thanks Dave, this makes sense now. My front rotors are 11" and my rears are 11.8". You're correct the fronts should be larger than the rear. I don't know why I didn't pay attention to this befor.
Thanks

Definitely worth hitting up ECI about it....Perhaps you ordered from a phone bank dummy and not a real tech????? The least they should do IMO is give you a big discount on the bigger brakes.....

I would suggest checking pressures with a brake pressure gauge....A good investment IMO....Maybe it's just the old circle burner in me, but I sure do like to know where my pressures are at, especially with a proportioning valve.... I even put them in the drag cars...Consistency becomes so important there we've discovered that even something as seemingly insignificant as getting the brake pressure right when staging helps..... Not as important with a transbrake, but even with a brake, never had a car creep the beam since I put a brake gauge in the car....

I know, I know....This is street, not a race car, guess I'm just competitive in all of them!!!!!!