Another brake problem - 4 wheel disc.

[oldman3]

OK, a short story. A friend dropped off his car for me to check why he doesn't have good brakes. The car is a 1957 Chevy Belair with a big block 427 and a big cam. I drove the car and his pedal is hard, but doesn't want to stop the car. It has power brakes, with disc master cyl and disc. combination valve. It does not seem to have any power assist at the pedal. He has a vacuum can in the front of car, and the highest I've seen is 15 HG. I see a couple things that I think are causing some of his problems, but would also like your input.

Pic 1 is of the MC and combination valve, for some reason he re-plumped the lines to the valve. He went below the valve and curved up above and back into the valve. The picture doesn't show it, but the curve in the line is 2" above the MC cap. Wouldn't this cause a large air pocket??

Pic 2 (not the best pic) He has 3/16" lines going to the front brakes, but also has a 3/16" line going to the back feeding two 3/16" lines to calipers. Should this be a 1/4" line feeding the two 3/16" lines??

Pic 3 Has metric calipers up front. These should work OK, but I think they should be larger???

[oldman3]

Pic 2

[oldman3]

Pic 3

[oldman3]

The last item is his power brake booster. With the lumpy cam he added a vacuum can to catch vacuum. In the pic the line with the check valve comes from the engine to the can (line is approx 5 feet long). Line with the brass fitting goes to the booster which is about 3 feet long. Highest I've seen at the gauge is 15 HG, don't you need 17-21 HG to the booster to work right. The booster is a 8" single diaphram unit, any help is greatly appreciated....Jim

[oldman3]

Out of 61 views, no one has a comment, am I on track or not, would like some feedback...Jim

[wraprail]

I don't think the line above the master will cause an air pocket. Street rods and many early trucks have the master under the floor with lines above the master. That said, I would not have run it that way. Those under floor master systems do require a 2lb. RPV on disc brakes to prevent fluid drain back which causes a low pedall which you don't have.
The 3/16 brake line is prefered, even on the rear and the T is the way to do it. No problem there.
Sounds to me like a lack of vacume. I'm not a fan of power brakes and don't use them on any of my cars or trucks. You get so much more road feel with manual brakes and in my opoion they are much more fun to drive. As long as you have the proper master bore size and proper pedal ratio. Thats what I would do. Lose the booster, get the right manual master for 4 wheel disc and replumb/relocate the combo valve. Also, add a adjustable prop valve on the rear brakes it you don't currently have one.

Peace....

After looking at your photos those hard lines look bigger than 3/16. Could be the photo quality or the angle. If they are 1/4 I would change them.

[oldman3]

wraprail, thanks for the reply. The two lines from MC to combination valve are 1/4" lines, the rest are 3/16". Any system that has the MC on the frame I've always used 2lb RV to disc and 10lb to drums, to avoid drain back. At this point I'm going to tell the owner what I've found, and let him decide what he wants to do. Thanks again...Jim

[ERASER5]

With engine off (after a small warm up to build vacuum) you should be able to hear the vacuum his as your helper depresses the brake pedal.

I had a booster problem once. replaced the booster and had the same problem. After pulling my hair out, I checked the only part that did NOT change. The check valve. A mud dobber had built a nest during the few days it was disconnected. That was an expensive mud dobber!

Second, are you sure there is not residual valve? A disc/disc MC should not, is it is the correct part. Some disc/drum MCs look very similar. The valve would be located in the MC outlet port for the rear.

[oldman3]

Erazer5, Thanks much for your input. Both are valid points. I have already checked the booster check valve and it is good. I also check the booster and it is not leaking. I think the big problem with the booster is not enough vacuum. I will talk to owner about manual set-up or buying an elec vacuum pump and get rid of canister system. I also thought about the 10# RV in most combination valves for drums in rear. The owner said he bought the booster, MC and combination valve from CPP, for a DISC/DISC set-up. He also got both the front and rear disc brake kits from CPP. I have also double checked to make sure the combination valve is centered and not closed to one system. Thanks again...Jim

[gimmy 64]

put a dual booster on it. i don't remember the numbers but read where you get more power from dual. i am only running a bout 15-16 lbs of vacume and my brakes work fine

[oldman3]

gimmy64, absolutely correct on the dual booster. I run a dual booster on my 50 pickup. Wish I had met this guy before he did the work to this car, I think I could have helped him alot. I'm NOT saying I know everything, but I'm 66 yrs old and have ran across lots of problems. Since this internet thing as been around I've LEARNED alot from people on here. Thanks....Jim

[jd7]

I had your problem, when i first got my truck the PO used a single diaphragm booster. the pedal was rock hard but no brakes. It held vacuum but did not stop worth a crap.When I built my motor and tranny I went with a duel diaphragm booster. Wow what a difference

[oldman3]

jd7, yes a dual diaphragm is the way to go, just have to talk the owner into changing that, and a few other things. Will let you guys know what he does and what the outcome is...Jim