Thread: Low Power brake booster vacum

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Denny,
It is rubber hose, I did not know that they had PVC hose?? is it PVC Vacum hose ?

deny's right about the hose, if you have 17" of vac, at idle, and you still have a hard peddle, then you have the wrong hose, bad one way valve or a bad booster. take the hose off the one way valve on the motor side and see if it holds vac. in the booster, or see if you can blow through it both ways.

Thanks guys, I will get the correct hose, recheck the one way valve and check the power booster to see if it holds vacum. I will keep you up to date. Thanks again.

Can't tell for sure from your picture, but it looks like the vacuum fitting you're using on the carb might be to a port that is above the throttle butterfly. This won't give you any vacuum at idle. The hose looks rather small, too. Have you put a reducer in the line to match the size of the hose to the fitting on the carb? The vacuum line needs to get full manifold vacuum, which is usually taken from a fitting on the intake manifold.

Hot Rod Roy has a point, that port is for PCV and may be a bit undersized for power brakes. Find a source of manifold vacuum. There should be plugs in the manifold somewhere.

Here's a few more pictures to give you an idea where I have the vacum line hooked to. As you can see it is at the base of the carb. In one of the pictures you can see a brass plug screwed into the intake manifold. Is where I have the 3/8 plug where the vacum line should be hooked up? I went to the auto parts store and they gave me vacum hose 11/32 that is made for power brake boosters.

start the motor and pull the hose off and put your finger on the end of it. if you have vac. then it hooked up right.

I removed the vacum hose while running and there is vacum. If I leave the hose off the idle goes up but the engine does not die. Does this mean I have another problem I should be looking for ?

Originally Posted by Jim Standley

I removed the vacum hose while running and there is vacum. If I leave the hose off the idle goes up but the engine does not die. Does this mean I have another problem I should be looking for ?

no thats pretty standard.

Well I found part of the problem, or a combo of problems.
1) The correct vacum line helped.
2) When I installed the master push rod that attaches to the brake pedal assy, I did not line the rod up so it pushed in at the correct angle. Once I corrected this the hard pedal became more free but still a little firm/mushy. I will rebleed the brakes and give it another try. I will let you know the out come. Thanks for all the help. have a great X-mas.

Here's the update. I bleed the brakes again and still not much more improvment. I put the car on jacks started it, put my foot on the brake pedal and had someone turn the tires, the fronts grab good but the back do not. I put it in gear so the back tires turned and applied the brake, the rear tires continued to turn. I checked the rear drum brakes and all looks good. The master, proportioning valve,brake lines and wheels cyclinders are all new. Could there be air trapped in the proportioning valve ?

Jim, do you have a 10 lb. residual pressure valve in the line to the rear brakes? If not, you'll need to install one. That will firm-up the pedal. Also, you need to adjust the shoes out against the drum so that you can just turn the rear tires by hand and get maybe a quarter turn by pushing on the tire and letting go.

I have taken the time to read your whole story and maybe this will help. First of all a 7 inch power brake booster is rather small for your ride. A dual 8 inch recommended but you could even go with a nine inch dual if you wanted to. I would use nothing smaller than a 8inch dual. It looks like the master is mounted on the firewall and if so you dont need any residual valves anywhere on the system. The only time you need residuals is if you master is located under the floor and is below your calipers. Use the correct proportioning valve though. Disc front drum rear if that what you have. That is critical. Also make sure that you have no more than .008 to .012 clearance on the push rod at the master. Make sure that your master is for disc, drum and the bore size is 1 inch or better I have 4 way disc and mine requires a 1 1/8 bore master. I think that you shoould now have all the answers for any possible situation. A vacum pump isn't going to help anything unless you have all the basics done right first. I hope I didn't sound like a know it all in writing all this. But I have pulled my hair out many times till I finally got it right.

Posted by Roy:
"you dont need any residual valves anywhere on the system"

You'd better call Master Power Brakes and tell them they're doing it all wrong then.
http://www.mpbrakes.com/uploads/docu...dualvalves.pdf

have same one pump the brakes and open the rear bleeder, to see if any brake fluid comes out. if not you have a restriction somewhere. you shouldn't need a residual valves if the Master slenderer is above the wheel cylinders. sounds like your proportioning valve is not centered.