RogerC

Let me tell you my tale of woe. 1970 Delta 88, 350 2 barrel, disc front, power assist.
The brake warning light came on and the back reservoir was low. Off the car goes to our trusty mechanic. We replaced the rear slaves, the master and the booster. The car was fine, but the pedal seemed hard. After a little research, it seemed like he might have put the booster for the "A" body on. A new booster was installed and now the pedal was rock hard. The car stops, but it is a very hard pedal. The vacuum gauge shows plenty of vacuum from the hose. Brakes have been bled. shaft for the booster has been adjusted. All to no effect. I think he tried a second dual diaphragm booster with no effect.
A hard braking test on a deserted road shows the car stops, no pulling but very hard pedal effort.

If the car is off, with the pedal down, and then the car is started, the pedal does NOT suck down. All symptoms point to the new booster(s) not working. I am about to order booster # 4 and install it myself. Unfortunately, the original booster was sent back for core charge and is not available. The brakes work, but they don't feel like power assist. If grandma was driving I don't think she would get it stopped.

I notice Rock Auto shows a booster for disc front, with a different one for drum front. I am hoping it's just a wrong part.
Thanks for any help,
roger.

svnt442

Pull the line going to the booster with the car running and make sure there is good vacuum there. If so you could have gotten 3 bad boosters, but the possibility of that happening is pretty slim.

MDchanic

I don't understand why the booster was replaced in the first place, for what sounds like a m/c problem.

I agree that it sounds like the booster isn't doing anything - I'd go through the checks suggested in the Chassis Service Manual.

- Eric

RogerC

As usual, it was the painfully obvious. I pulled the vacuum hose to the booster to check vacuum, ( which was there) and thought "hmm, I bet the hose isn't supposed to be split" I put a new hose with new clamps, Presto! zoot! Allors! Brakes. Thanks to all. Now the next item I'm off to the defroster section.

svnt442

And I bet the car is running tons better now too!!

RogerC

Thanks, we are getting there. I came back this a.m. in search of something like a heater control vacuum line schematic. The front defroster is not defrosting.

Rickman48

Look for the cracked vacuum line under the hood that goes into the cowl - or listen under the dash.

MDchanic

I think you're looking for this diagram, which is located in the Chassis Service Manual here.

You should join WildAboutCars (free) so you can get these manuals online until you get your own set.

- Eric

RogerC

I'm a little late but I have another report. The cracked vacuum line was part of the problem, but in a "what can it hurt to try?" moment my new mechanic and I tried another new brake booster. NOW it has brakes. It turns out there were two bad rebuilt brake boosters in a row, and the third time was a charm. Mechanic suggests that the booster can be checked before installation by hooking the vacuum hose to the booster and working the brake rod.