78SC/Brake Booster O-Ring/Master Cylinder Question

[lespaul]

I found a pool of brake fluid on the driver's side floor. Inspection of the master cylinder/brake booster area showed no leaks from the brake switches or anywhere around the master cylinder except right below the junction of the MC and the booster. Looking at the diagrams on line, it appears that there is a replaceable O-ring between the MC and the booster.

Questions:

1. Is it likely that replacing the O-ring will stop the leak or should I replace the entire master cylinder? $200 vs $5.

2. If only replacing the O-ring is necessary, can I easily just remove the booster from the car and leave the MC in place to do that job - or do I need to disconnect the MC as well?

Thanks guys.

[Josh D]

Your MC is shot and needs to be replaced. Your booster is filling up with fluid. I had the same issue and only replaced the MC, only to have the booster fail two days later.
The booster has to come out to get the MC out. Not a terribly difficult job.

[lespaul]

Thanks Josh. Was your booster failure just coincidental or was it caused by the fluid coming from the MC into the booster?

Brad

[DRACO A5OG]

Place a mirror between the booster and MC. if you see melted black paint from the two then down to the chassis, then chances are seal is compromised.. Try that first. if it still leaks, then get yourself a New Good used one.

[lespaul]

DRACO,

Sorry, I am confused. When you say "chances are seal is compromised" are you talking about the o-ring and suggesting I replace it before putting a new MC in?

Brad

[DRACO A5OG]

^^^ Yes, if you see no melted paint then previous poster may be correct MC is shot.

Check the area first.

Seal = O-ring. Host sells it, a common problem due to age.

[LJ851]

Originally Posted by DRACO A5OG

Place a mirror between the booster and MC. if you see melted black paint from the two then down to the chassis, then chances are seal is compromised.. Try that first. if it still leaks, then get yourself a New Good used one.

You are thinking about this wrong. The purpose of the o-ring is not to keep fluid in, it is a vacuum seal.

If there is fluid coming out between the master and booster, the only cause is a defective master cylinder that needs to be replaced. The brake fluid entering the booster can also be hard on the rubber parts in the booster as mentioned above.

[DRACO A5OG]

I agree 100% the o-ring is a Vacuum seal. OP may have a leaking MC.

In my case, the MC was leaking at the plunger then down to the o-ring, compromised it but did not create a vacuum leak and over time leaked on to the booster housing as evidenced by the melted black paint from the connection down to the chassis.

I replaced the MC and brakes were not only solid again but the housing was dry.

[Josh D]

Originally Posted by lespaul

Thanks Josh. Was your booster failure just coincidental or was it caused by the fluid coming from the MC into the booster?

Brad

The fluid is corrosive once its exposed to moisture. It compromised the diaphragm seal between the two booster halves. The compromise was masked by the puddle of fluid at the bottom of the booster, so it still worked without a vacuum leak. Once all the fluid was cleaned out and dry, the leak was exposed.

If you've caught it early enough, you maybe ok. It just sucks to get it all back together and then find the booster has failed. New ones are very hard to find as well. Parts wharehouses will show reman listed, only to show out of stock when ordered. I ended finding a used one in perfect condition.

[lespaul]

Thanks All. I have ordered a new MC -- it will be here Friday and fully installed by end of day Saturday. I will let you know how it all comes together.

Brad

[DRACO A5OG]

Let's pray it is just the MC, when you have it apart inspect the booster inside. If dry you lucked out.

If compromised contact BLACKBYRD, he has couple of good ones.

[lespaul]

Update:

I took the MC and Booster off last night and separated the two. The Booster had about a cup of rusty brake fluid inside that had been leaking out of the bottom where it joins the MC.

I poured alcohol inside the Booster and swished it out several times to get any brake fluid out. The Booster may be shot but, since they are expensive, I decided to re-assemble with a new MC and see what happens. I should have it back together on Saturday.

Meanwhile, this gives me a chance -- while I am in there -- to rebuild my pedal cluster and install my Rennline aluminum floorboard.

Brad

[lespaul]

So, I installed a new MC and, while I was at it, new bronze pedal bushings (that roll pin is a b---h) and new pedal covers. The hardest part of the job was reconnecting the clutch cable to the pedals with that crazy pin that is impossible to reach way way back in the guts. An hour of cussing and struggling.




Bled the brakes using new speed bleeders -- nice purchase -- and took it for a test drive. Good brakes but the hisssssssss still emits from the Booster when the brake is depressed. I think the Booster is compromised, but working. Plan to replace it with a reasonably priced one. Might go to the Hershey swap anyway.

Brad

[jjeffries]

Brad et al, is there meant to be a cover or grommet over that front tunnel access opening, in your photo just ahead of the shifter? None on my 82, suppose it's just covered by the carpet and the console? Thanks, John

[DRACO A5OG]

No, there is no cover or grommet.

[lespaul]

I put a Rennline floorboard in last night -- pics later. My plan is to cover the floor and the center console and about 4 inches up the sides with rubber diamond pattern flooring material that comes in rolls. No carpet and no floor mats.

Brakes are holding but the booster is not doing its complete job so I need to press harder than usual.

[Josh D]

Is the vacuum leak when you are holding the brake pedal not causing idle problems? Mine was bad enough that it would stall.

[lespaul]

No idle problems. Steady as she goes. Just a hissssss when brakes applied and, it seems, more force is needed to brake than usual.