How do I diagnose a bad brake booster on an 89
 

I see a fluid leak from the bottom of my master cylinder, my brake fluid level is OK, it's really just seaping from under the cylinder down the booster. I know I need a master cylinder, but are there any tests to diagnose if I need a booster. I hear the booster is difficult to replace. And what economical brand cylinder or booster can I buy?? Are rebuilds OK??? How hard is it to replace the booster??JZ

Gee... lotsa questions...

If the booster is bad, three things will happen 1) it will hiss. 2) it will loose vacuum pressure just sitting. 3) the pedal will get hard to push.

The leaking fluid is a bad master cylinder.

Replacement? I haven't done that job yet... donno.

Parts? see above.

If its leaking from that weephole on the booster, then you should replace the booster, which is what I'm doing this weekend along with my MC.

And yes, replacing the Booster is a bitch.

Run a search for my recent threads and you'll find a link to where I got my booster.

It is leaking from right under the master cylinder where it connects to the booster. I don't hear any hissing but the pedal is alittle hard. But couldn't a hard pedal be from an adjustment??? Any better ways of testing booster and where exactly is booster weep hole?? JZ

the weephole on mine is directly under the MC, small indentation directly below

you will need to replace the MC, but if the pedal is not too stiff and you hear no leaks its nearly impossible to tell if the booster is going out

Can a bad master cylinder cause the pedal to be stiff?? I read a thread that said to pull vacum tube cap on booster and stick a rag down into it to see if there's brake fluid inside, can doing this damage the booster?? JZ

No, a bad master will cause the pedal to be soft.

A bad booster will cause it to go hard.

Damn... the double entendres are running rich today. :D

If the master cylinder is bad, it will pour brake fluid into the booster, but that is not the sign of a bad booster.

To review - bad boosters = hard pedal, hissing sound from booster, sometimes, it will also lead to bad engine operations, due to a large vacuum leak.

Another thing to check - literally - is the check valve on the booster. It will go bad... and will lead to all sorts of wacky problems.

With regard to the original question - here is an easy way to determine if your booster is bad (if you don't have an obvious hissing). With the car not running, press the brake pedal four or five times. This will get rid of any residual vaccuum in the booster. At this point the brake pedal should feel real firm when you press on it.

Then, with your foot on the brake, start the engine. You should feel the brake pedal go slightly soft and depress further without you putting any more pressure on it. That's the effect of the booster. If you don't get any change in the brake pedal feel, or not enough, when you start the engine, you have a bad booster.

As with anything safety related such as brakes, it is probably best to replace any part that you even suspect has gone bad.

I didn't think replacing my booster was that difficult, but it does involve getting your whole head and shoulders in the relatively narrow driver's footwell. The right tools also help - I used a swivel socket at the end of a 6-8" extension for the part under the dash. Also, mine is an L89 and LT1's may have more stuff in the way of the booster itself in the engine compartment.

Let us know what you find out. -Dave

Dave,
Thanks for the info, I will test it tomorrow. How can I tell if the booster valve is bad??? JZ

Reviving a really old thread here, but I'm having some brake issues and am not entirely sure what I need to replace. Some background info: when I got the '84 (120k on it), the brake fluid was really opaque and muddy brown and the brakes didn't work too well. When I tried to drive it to the auto parts store, the brakes locked on and didn't release until I pulled over and let the car sit for a minute. I replaced the fluid in the reservoirs with new brake fluid and then bled the lines (boy was that a LOT of nasty brake fluid that came out!). Now it seems that when I first start the car, the brakes are really weak and I have to press pretty hard to get a significant response, but after I've driven for a few minutes and used the brakes, they get more responsive. I have noticed a hissing sound when I press the brake pedal, and I've checked that there's vacuum going to the booster and it appears the check valve works. Right after I bled the brakes I noticed the pedal depress when I started the engine, but it doesn't seem to do that any more. I'm thinking the booster is going/gone based on the hissing sound. Any thoughts on what could be going wrong and/or how to check it?
Thanks,
Reed

I just went through his had the hissing when I pressed the brake pedal. The drive home was essentially manual brakes. Also the vacuum leak was causing a slightly high idle.

I changed the booster and all was well again.
I got my booster of Amazon. it was a reman by AC Delco. They bought the old one back for $40 (essentially a core) but they do it different. So Final cost was $110. They pay the shipping back and once they get it they send a check.

As far as the change, it's not too bad except for the brace from the hood latch to the frame.

I chose not to climb under the dash, I'm to old and to fat for that.
I lowered the steering wheel which takes around 20 minutes and could remove and install both nuts using 2 extensions.

AutoZone has a metal replacement for the stock plastic booster I used.

Couldn't be happier. Plastic one cracked from top to bottom

Yeah, I'm considering replacing the booster just to be on the safe side. Any idea if the master cylinder is bad? Don't see any fluid leaking, but with the amount of gunk in the brake fluid, it wouldn't surprise me if the cylinder is gummed up somehow too.

When they're bad you will typically see fluid leaking past the piston in the rear, or you won't have adequate pressure.

You can flush it out with new fluid if it works okay.

I have noticed the new fluid is starting to get cloudy again, but I suspect it's just cleaning some more of the residue and crud from the system. I'm planning on flushing/bleeding the system again soon once I've driven it around some more.
Thanks,
Reed

I lost my booster while driving in traffic. I thought I was going to crash as all of a sudden my brakes seemed to fail. But I really pushed hard and she stopped. My pulse rate went up a bit and I recovered. I bought my booster from Rock Auto and the installation was simple in theory but somewhat difficult in reality. I'm not the smallest guy around and getting under the dash for those pesky 4 nuts and that clip was not fun.

I have to do a booster since mine cracked last week. I see you lowered the steering wheel how did you do that and are you just taking about the 2 steering colum nuts?:flag:

So just an update, I replaced the booster and can verify that the old one was bad. Brakes are now nice and responsive (just about put my father-in-law through the dash when I first tested them) and I no longer have a hiss when I apply the brakes. It was a fairly easy fix and well worth the $115.
Reed

Once you get the lower panel and the metal reinforcement panel off, you remove the tilt handle for clearance and then the two bolts (at least mine are bolts), that will drop the column down and you can see the nuts for the booster.

I replaced mine a few days ago, I took out the seat(then had to clean) It is a bear to get the bolt out abovr the steering. I have not taken metal reiforce piece out yet, I will have to see how that works. Brakes are working better than before with a A! Cardone booster. The booster rod was preadjusted, I could not source a metal one in my time frame.