Brake Booster Problem

[ccz145a]

I just finished putting my truck back on the road after a cab swap. The doner cab was an 84 diesel that I bought for $300 with no engine/trans or rust. I have no idea what brake system came with the truck, but a previous owner had started a gas engine retro-fit and they had plopped on a brake booster that was never hooked it up to the pedal.

I realized the reason the pedal wan't hooked up was that the booster's rod was the wrong hole diameter for the brake pedal's pin, so after some careful measurement I hoged out the opening to fit on the pin. I also realized the pin on the donor pedal was an inch closer to the pivot pin, I assume to increase leverage for manual brakes.

Anyway, I ended up putting on a rebuilt booster of the type my '75 had (double diaphram), bolted it up. All is good, right? Wrong.

I push on the brake and hear a great whooshing sound, engine begins to stall like it has a huge vacuum leak. I Look under the dash where the sound is coming from and mash the brake - the accordion rubber boot on the brake rod is being sucked towards the booster. So I figure the booster is bad and return it to NAPA, they give me a new one, I put it on.

Same thing, different booster, same problem.

What gives?

There is a disk of jute material on the rod, was I supposed to stuff that up in the booster opening?

My old booster had a plastic covering that covers up the area between the booster and the firewall, I did retain this part and it is installed. The jute disk is inside this covering and the rod goes through it.

Is the fact that the pedal pin is higher on the pedal by an inch causing the rod to be at the wrong angle and rip the diaphram?

Could an engine backfire rip the booser diaphram? I had a burnt plug boot and the engine coughed a few times on startup.

Any help is appreciated.

Thanks.

[VileZambonie]

Did you install the check valve on the booster? Yes a backfire can destroy the booster. If you had a hydroboost system originally you need the pushrod from the vacuum booster system because I'm pretty sure they are diffrerent.

[ccz145a]

Yep, came with a check valve ( And a waranty void warning if removed ).

Might have had the hydroboost, since it was a highly optioned Silverado. Can't imagine that it had manual.

The pushrod is the same dimensions as the one from my 75, just enlarged the hole to 9/16" to fit the pin on the pedal.

What about the little jute pad disk on the pushrod by the booster, does it get stuffed up in the booster opening?

Guess the backfire did it. I will take it back and get another.

The really sad part is that the original 75 booster was working just fine, I was just getting a new one because I couldn't get a fresh coat of paint to stick. I mean, why clean and restore a 30 year old part that might break any minute. Oh well.

Thanks, Vile

P.S. Wonder if they make a double check valve to keep this from happening?

[VileZambonie]

yes they do. there were factory installed valves. I've never needed one myself but nail down your timing issue if you think that's what's popping your diaphragm.

[ccz145a]

nail down your timing issue

It was the plug wire. fixed. Thanks.

[Blazin]

Options such as Silverado have nothing to do with it being a Hydro Boost. All 1 tons had them, & all diesels had them 1/2, 3/4, & 1 tons.
Only other thing you will find Hydro Boost on is full size station wagons, Impala, Parisian, etc.

[Captkaos]

Chip, yours would have came with the hydroboost as Blazin indicated as it was originally a diesel.
The felt piece just goes where you put it.

[ccz145a]

**Update** Works great now.

What was wrong?

Well, the hydroboost has a return spring on the pedal. That spring messed everything up and pulled the pedal back way too far. The big sucking sound was the slack being taken up as I pushed the pedal.

Got a pedal from the trusty junk yard locally, derusted it, painted it, used the good bushings from the existing pedal, and put it in. Taking the old one out, I noticed the return spring that was not on the one I pulled from the junk yard truck.

NAPA has been real good to me. They took back both boosters that I said were bad with so much as a funny look. I will be sure to do my business with them as much as possible to make up for it.

Thanks for the tips.

BTW. The push rod pin location on the hydroboost pedal IS way higher than the vacuum pedal. But it would have worked just fine without the return spring.

[Captkaos]

Chip, Glad you figured it out.  I appologize for not calling you back.  I had a note to do so and never got a free chance...