Thread: Power Brake Vacuum

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I have a Power Brake problem. I think it's a vacuum problem. Now, I have been told you need 16-18 lbs vacuum to operate the booster. OK, dumb question. Is that in park or drive? I have 18 lbs in park and 14 1/2 to 15 lbs in drive. I can not get brakes on this car. I have a new booster and master cylinder ,all new lines, new rear wheel cylinders, new brake pads, rear drums turned, new front brake hoses, new calipers on the front, new rotors on front, new combination valve, everything new except 10 lb residual valve. Brakes have been bled numerous times. Still the brakes are no good. Any help would be greatly appreciated! Thanks

There are three things that come to mind, the first is that you do not have a vacuum reservoir such as the Crane Unit (see attached picture). The second would be the initial setup of the brake line distribution block and 10lb residual valve placement. Instead I suggest an adjustable proportioning valve (see second attached picture). Third would be the size of the Master cylinder bore (IE: 5/8th, 3/4, 13/16th, or 7/8th) and which combination of all three you may or may not have. Who sized everything up for your car and exactly what do you have installed on your car from the Master Cylinder all the way to the brake calipers/drums front and rear? Final thought would be what "throw" you actually have with your current pedal setup and is it sufficient to pull/push the piston in your current master cylinder and move enough fluid to build up any real pressure.

Will await an extremely detailed response and we can then go from there.

Ok, I have a 1938 Chevy 2 Door Sedan with a 700R4 trans and a 350 SB. Now, I don't know what the bore size is or any of that on the master cylinder. I bought a complete set up with Corvette style master cylinder and booster as one. I then replaced all the brakes lines and a new combination valve, new Camaro calipers on the front with new rotors and brake hoses. I replaced the rear wheel cylinders, brakes shoes and turned the drums on the Chevy 10 bolt rear. I moved the Master and booster from under the floor so it was easy to get to. So that's now on the firewall. The Brake pedal ratio seems to be OK. The only thing I didn't replace were the two residual valves one 10 lb and one 2 lb. The brakes were bled numerous times and with the same results. I have changed the master and booster three times thinking one was bad. That had now change in the pedal at all.I checked the vacuum for the booster and at idle in park it's 18 lbs. But, when I put it in drive it drops to 141/2 to 15 lbs. When I push the pedal it seems ok for a little distance and then it just seems to bottom out. Then it's like pushing against a brick wall. No one can figure it out. I have never had a problem like this one ever working with brakes. If you need more info PLEASE let me know. I am really stuck here! Thanks for your help.

Do you get full pedal travel when bleeding the brakes? Your description of the action wasn't clear on that point. If it doesn't go to the floor with a bleeder open, you have linkage problems.

Yes, I bled them the proper way. Yes, I used PVC hose and it's hooked up to the back of the carb. Is there a way to check that check valve? Will having the residual valves in affect the brakes if left there? Thanks!!

Yes, the brakes were adjusted correctly and I have a full drop on the pedal when the valve is opened.

As Denny said, check the rear brake adjustment...

Also, I would be curious to know what size the master cylinder bore is, how much the car weighs, and the diameter of the brake booster.....ie a 7" booster and a 7/8" cylinder won't stop a 3700 pound car very well.....

Yes, I purchased the master and booster together from Speedway Motors as a set. The booster is a 7" and master cylinder bore is 1".

The rear brakes are adjusted just so I can just turn the wheel. Maybe a half of a turn free.

Don't know if this is your problem but I've never been able to get decent brakes with a 7" booster, tried one a few years back. I run the 8" dual diaphragm on almost everything now, lot's better braking... The 1" cylinder should be ok up to at least a 4,000 pound car I would think....

The 7" booster IMO is going to be marginal at best, to help it along might want to try running the rear brakes as close to dragging as you can..... I still prefer to run the rear drums tighter (but not dragging) with a residual valve and a proportioning valve in the rear line.....

I will try adjusting the brakes again tomorrow. But, I think they are just about as close as possible to being right. Is there a way to check the check valve in the booster? THANKS!!

Have you checked the system with a brake pressure gauge to see what your pressure at the wheels is????

No, I don't have a brake pressure tester.

Might want to borrow or buy one to check the pressures with. I have heard of a combination valve sticking and bypassing pressure.....

Thanks, I will see what I can do. I really appreciate the help!!