Brake Booster Repair/ Troubleshooting

[James Lindner]

Hello All
I have an early MkI with the stock Girling Mk2A booster. It was completely rebuilt 10 years ago.

I recently had my brake pedal go to the floor while driving (nothing too serious as I was moving very slowly in stop and go traffic.) I immediately suspected the Brake master and sure enough, upon removal and disassembly it needed to be completely re-sleeved and rebuilt.

I got all that done and got the MC back in the car. During the test drive the brakes did not seem to be getting any help from the booster.

Since the test drive, I performed Randy’s “old School” test and when I started the car with the brake pedal partially depressed it DID NOT move forward at all. Conversely, with the car off I pumped the pedal several times and the pedal strokes never shortened and continued to travel the
full range which I interpreted as evidence it is NOT holding Vacuum.

I tested the manifold vacuum and got 12-13 inches @ idle (700 rpm.) The fluid level in the reservoir does not appear to be dropping, so I do not think it is blowing by bad seals in the
booster.

My question…is there any intermediate repair (check valve, gasket etc) short of a complete rebuild of the unit.

Thanks

Jim

[Tom and Joanne Ehrhart]

As you noted, seems like servo is not actuating. But Why. there are two basic reasons.
1. Vacuum, or lack of.
2. Servo vacuum piston is not actuated.

Test #1 Let’s eliminate #1 as possible cause. Start engine, unscrew bolt a turn or two securing the vacuum check-valve banjo fitting. There should significant air leak and the engine idle will be affected. That’s good. This means you have Vacuum at the booster.

Test#2. Engine off. Remove 4 Phillips screws securing servo side valve cover. Pivot away so the lever in side is visible. Have a helper step on brake peddle (eng off). You should see the lever move a small amount. If the lever does move. Then I suspect the vacuum piston is too tight in its bore (see below). If the lever does not move, return to rebuilder and have it repaired correctly. I doubt this will be a problem.

Vacuum piston seal.
The primary reason is that the leather seal sticks to the bore and fails to activate. There are several reasons for this.
1. I have had a lot of trouble with recent repair kits related to the rubber tubing used to “set” the leather seal. It tends to put excessive pressure on the seal causing too much drag. In effect sticking to the cylinder. Most rebuilders don’t have a clue about this issue.

2. If any moisture accumulates inside the vacuum canister, slight surface rust may occur, especially adjacent to the leather seal, which will cause it to stick. I don’t care how shiny it looks outside.

3. Improper leather lube, or no lube was used in assembly. Again, causing leather to stick to the bore.

Here’s a crude possible fix if the seal is “slightly” sticking and #1 has proven that you DO have a vacuum at the servo. Start engine. Have helper apply normal (like sitting at a stop sign) pressure on the brake pedal. While pedal pressure is applied, use a heavy plastic mallet with gusto to sharply hit the edge of the canister (where gasket is visible between the two plates).

If the piston is lightly stuck, it should free itself. Actuate the brakes a few times to exercise the piston.

If this does not solve the problem, send it back to the rebuilder, or take it apart yourself and find ot why the piston is sticking.

TT

[James Lindner]

Just to close out this topic…

I checked the vacuum as described by TT…it was fine. I do not think the fluid is leaking past the seals because the fluid level is not dropping.

Offline TT suggested I readjust rear brakes as that might improve the pedal range. I did that IAW the WSM.

I took it for another test drive to test the brakes. The brakes performed well even under panic conditions. The pedal range also seemed to improve.

At this point, I am not convinced I had a booster problem. Might be a slight case of “chicken little.”

Jim

[James Lindner]

Just to close out this topic…

I checked the vacuum as described by TT…it was fine. I do not think the fluid is leaking past the seals because the fluid level is not dropping.

Offline TT suggested I readjust rear brakes as that might improve the pedal range. I did that IAW the WSM.

I took it for another test drive to test the brakes. The brakes performed well even under panic conditions. The pedal range also seemed to improve.

At this point, I am not convinced I had a booster problem. Might be a slight case of “chicken little.”

Jim

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