[TheCiscoKid]

In a 38 Chev,I used a 7'' stainless DUAL diaphram booster,chrome 1'' reservior, firewall mounted with S10 pedal,GM proportioning valve, disc front, drum rear.Engine has 18hg.The brakes have never been as good as I hoped.I think my other motor had more vacuum,but brakes were the same.While driving brakes seem fine,altho haven't tried to lock them up at any speed, but at low speed brakes feel like they don't grab, a hard foot will not lock them up.Do I have a brake problem or is this just how brakes are with a 7'' booster? thanks

[aosborn]

Do you have alot of travel on the pedal?

With what you describe, I would suspect the factory proportioning valve to be an issue. A firewall mounted pedal usually means a nice long pedal and lots of leverage. That being the case, I would typically go with a 1 1/8" master cylinder and an adjustable propotioning valve in the rear line along with a 10 lb residual preassure check valve. The dual 7" booster is a great unit, and you should have plenty of preassure to lock up the wheels with ease.

I ALWAYS recommend using 11/32" vacuum brake hose to hook up a booster. Using anything of lesser integrity you run the risk of the hose collapsing internally, especially if it is a long length of hose.

[A56Nomad]

Check out the miles of print on Chevy brakes on ALL sites. I have a 74 Chevy that has been that way sense NEW. I have even talked to a brake mechanic from Chevy, he stated that he had the factory rep out and couldn't stop the truck on a dime; he just stated "No extra charge for the long stop" as he left!! Chevy knows that they have a design problem with several years of their trucks.
That is just the way Chevy brakes are. As the last person stated, get rid of the proportioning valve and use the adjustable valve and a pressure regulator and it should work.
I went with ceramic pads on my front rotors and it helped allot. I will change out all the Chevy brake items and go with a totally new after market system, that may be the only way to eliminate the not so fast stopping Chevy brake design.

[aosborn]

I am going to put in a plug for Carbon Metallic brake pads. If they are available for your application, I highly recommend them. Great stopping performance even when cold.

[TheCiscoKid]

what does the residual valve do, thought they were used just if master cylinder was low mounted ,to hold pressure?

[aosborn]

The 10 lb residual pressure check valve is used only for axles with drum brakes. It keeps the wheel cylinder pistons expanded against the push-rods to the brake shoes. If you don't have one, the return springs on the brake shoes will compress the wheel cylinder pistons and the next time you push on the brakes, you will need alot of pedal travel to apply the shoes. The check valve can be built in to the master cylinder, or it can be installed in the line going to the drum brake axle.

The 2 lb valves do just what you are talking about. They keep the fluid from running back to the master cylinder when it is lower in the car than the calipers. You do not need one for firewall mounted master cylinders. In the same situation with a low mounted master cylinder the 10 lb valve for drum brakes suits both purposes. Use a manual adjustable proportioning valve in the rear line also to be able to adjust brake balance.

[fiscus]

This is a longshot and probably not your problem, but something to consider as a possible factor: It could be your pads/rotors. Try scuffing the pads and rotors with some 100 grit sandpaper, or change out the pads for a new set. Supposedly the cheapest pads are the ones that bite the hardest