Vacuum and power brakes?

[garold99]

Ok, I have a '71 with power brakes. Disk on front, drum in the rear. Little over a year ago my breaks started getting weak, I would have to pump them to build pressure 2-3 times. So I did the test to check the Power Booster, the one where the engine is off and you pump the breaks up then hold peddle down and turn engine on an if break pedal goes down some then the booster is good. I think thats right.

Anyhow since that was ok, I figured the Master Cylinder was bad. So got a new one, bench bled it (without a bench), and put it on. But still my pedal pressure was on the weak side and I would have to pump the breaks a couple times. Been this way since I put the new master cylinder on over a year ago.

So I was thinking, there is a vacuum hose running to the power booster, maybe there's a leak and thats the problem. But the big hose looks good running to the little T connector behind the carb. But there is a hose on the opp side of the T connector thats about 1/4 inch diameter hose roughly that looks fairly deteriorated. The hose is a few inches long and connects from the T connector behind the carb into a metal hose about 1/4 inch diameter roughly.

So my question, do power breaks use vacuum? And would this litte 1/4 inch (roughly) hose have anything to do with vacuum to the power booster?

Thanks

[john]

The small hose goes to the vacuum modulater for the tranny. If it is bad the trans will shift at higher rpms. Another thing that will cause you to pump the brakes is the rear shoes out of adjustment.

[SactoJim]

I'm no brake expert, but from what I know......

The vacuum canister only assists with braking and makes the peddle easier to push on and apply the brakes. Without vacuum assist, the pedal would be very stiff and harder to push to apply the brakes.

It you still have to pump the pedal, you must have a weak brake cylinder somewhere in the system that is allowing brake fluid by it and/or air in the lines as well.

[garold99]

The rear breaks have never been great in this truck. I couldn't even tell you if they actually even work. Every time I pull the drums the breaks look like they have no wear on them. So last time I pulled the drums about a year ago, I rebuilt one side, but never got to the other side. Guess I will have to do that and see if thats my problem. I do know that before I rebuilt the one side the adjusters were frozen. I better get after the other side and check it out. Don't drive it a whole lot so wasnt real worried about it. But have rebuilt the front disk breaks completely. What I would really like to do is convert the rear drums to disk. Not sure if my knowledge goes that far though, or my bank account right now.

On the plus side though, the tranny shifts fine so I guess the hose for that is ok.

Thanks

[dougs69c10]

Originally Posted by garold99

Ok, I have a '71 with power brakes. Disk on front, drum in the rear. Little over a year ago my breaks started getting weak, I would have to pump them to build pressure 2-3 times. So I did the test to check the Power Booster, the one where the engine is off and you pump the breaks up then hold peddle down and turn engine on an if break pedal goes down some then the booster is good. I think thats right.

Anyhow since that was ok, I figured the Master Cylinder was bad. So got a new one, bench bled it (without a bench), and put it on. But still my pedal pressure was on the weak side and I would have to pump the breaks a couple times. Been this way since I put the new master cylinder on over a year ago.

So I was thinking, there is a vacuum hose running to the power booster, maybe there's a leak and thats the problem. But the big hose looks good running to the little T connector behind the carb. But there is a hose on the opp side of the T connector thats about 1/4 inch diameter hose roughly that looks fairly deteriorated. The hose is a few inches long and connects from the T connector behind the carb into a metal hose about 1/4 inch diameter roughly.

So my question, do power breaks use vacuum? And would this litte 1/4 inch (roughly) hose have anything to do with vacuum to the power booster?

Thanks

did you bleed the brakes at the calipers and drums when you swapped the m/c?

[Longhorn Man]

Locked up adjusters would cause this problem.

[garold99]

Yes, breaks were bled when MC was swapped. I completely rebuilt the passenger side drum, with the exception of a new drum. Never got to the drivers side though. Could be the problem. I did change the front disk breaks a month ago, but they still looked good when I did.

[junkyardjohn]

I HAD THE SAME PROBLEM WITH MY 71 BLAZER. BACK BRAKES LOOKED GREAT, SO I REBUILT THE FRONTS (NEW ROTOR ,CALIPERS & PADS) ALONG WITH A NEW MASTER CYL. I BLED A 1/2 GALLON OF FLUID THRU IT TRYING TO GET A GOOD STIFF PEDAL. I FINALLY REPLACED THE REAR WHEEL CYL. & SHOES... & WALAAH .... GREAT BRAKES. JOHN

[garold99]

Guess I'll be working on the rear breaks. I really would like to have this as a daily driver again soon. Just cant stand always having to pump the breaks. Thanks for the advice everyone.