Problems w/Brake Master Cylinder installed in '71 Trans Am
 

Good Evening,

I recently had a master cylinder that I purchased from Firebird Central installed in my '71 T/A (power front disc; rear drum) by a friend of mine that has a good deal of experience working on old Pontiacs (I have none whatsoever). However, once installed, the car had very little breaking power, along with a very stiff brake peddle. In addition, when the engine is running without brakes applied there is a slight hissing sound emanating from the master cylinder, which gets quite a bit louder when the brakes are applied.

Now my friend who did the installation thinks perhaps it's not the correct master cylinder for the car. Also, he pointed out that on the bottom of the the cylinder, near where it mounts to the power booster, there are two holes that face to the right and left of the car, respectively. And when you plug these holes with your fingers while the engine is running, the hissing stops. (Perhaps these holes indicate some application for another type of car, and therefore, evidence that this master cylinder is not the correct one for my car?)

At any rate, if anybody can give me their thoughts on what might be the issue with this master cylinder, I would very much appreciate it.

Thank you very much in advance!


CEB

pics might help but it sounds like some sort of booster leak? do you have a vacuum gauge?

Hi Steve,

I don't have such a gauge, but I'll ask the guy that did the install. Also, I'll try to post pictures, when I next get to the car (it's currently with my mechanic friend). in the meantime, here's a link to the Firebird Central page from which I ordered the master cylinder. The one I purchased (Stock #BRC-432b) is about 12 rows down the page.

https://www.firebirdcentral.com/FIRE...INDER_s/48.htm

Thanks much for your reply.


CEB

I should also mention that the master cylinder is the type with "bleeders," in the event that this further info may help in figuring out the problems.


CEB

maybe those holes are for hideaway headlights on some cars? you could check the napa site to see if the mc they sell for your car also has those holes. if so, maybe they just need to be plugged?

Thanks for the tip. I'll check the Napa site.

Regardless, if there is audible hissing and a firm pedal, that's a pretty clear indicator that you have a brake vacuum leak that is causing the booster to be less effective.

Got it. Thank you, ZeGermanHam.

I have never, ever heard of a master cylinder that's vented like that.

What vehicle would that be used on? For what purpose?

Is this a Delco; or a Bendix master cylinder? Something else? Does it LOOK like the one that came off, or does it look completely different?

They aren't vents. They are bleeders for the master cylinder. Sounds like the OP either has a bad booster or a vacuum leak.

What was the reason for replacing the master in the first place?

Do you mean the holes circled in red, or highlighted in yellow?

The bleeds are located directly above the brake line ports on the driver's side and have bleed valves in them. What the OP describes is something else, though by his description, I don't know what he is referring to besides the mounting holes in the ears of the flange.

OP, you have a brake booster vacuum leak. The booster is sealed and normally doesn't leak whether there is a master cylinder mounted or not. The mater cylinder does not see engine vacuum. You have a leaking brake booster. If the hissing is coming from the holes in the mounting ears, you leak is coming from around the mounting studs.

If the master does have holes in the bottom, I can't imaging what purpose they would serve. The body of the master cylinder seals tight enough that your internal booster leak is telegraphing the vacuum signal through those holes.

Short but sweet ... hissing, loss of vacuum and hard pedal is a leaking booster

Not possible.

He's not complaining that the booster is sucking the fluid out of the reservoir, and the bleeder screw holes would have a direct path to the fluid in the cylinder, and from there to the reservoir. In addition, the bleeder screws wouldn't be OPEN so that they could be plugged with his fingers to stop the air leakage (hissing.)

I want photos of this hissing master cylinder.

Schurkey - I'm not sure of the make of the master cylinder. I assume it's a reproduction - I purchased it from Firebird Central, and they don't state the manufacturer.

Steve25 - with the old master cylinder, the brake pedal went near to the floor when applied, and braking power was almost non existent.

i82much - the holes are the ones that you have highlighted in yellow.

As to everyone, thanks very much for your input. It sounds like my issue is the booster and not the MC, though the holes that are highlighted in yellow as shown in i82much's picture of a MC are, I believe, like the ones that I covered with my finger tips, and which when I did so stopped the hissing sound.

I should mention that prior to having the new master cylinder installed, there was no hissing sound, either with the brakes applied or not.

First and foremost it does not sound at all like the new master is the problem as there is no way for vacuum from the booster to get into it!
You issue lies in the booster , the hose that feeds it or the plastic fitting or the rubber grommet that the fitting presses into,

Thank you steve25. It sounds like it's time for a new booster, or a rebuild of my old one, if possible.

Possibly the brake pushrod in the MC/Vacuum booster is wrong length?


:confused:

boosterdeweyexchange.com