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Brake Servo

My brakes just didn’t seem to work that well especially considering they’re upgraded. I also noticed that my fuel injected, ecu driven engine was not idling perfectly, and I had an irritating surging from the engine at steady speed on light throttle. This all seemed to point at a leaky brake servo. This was confirmed by doing a few tests:-

Test 1

With the engine off, pump the brake pedal a few times to push the brake pistons against the disk/drum. The pedal should now be hard. Start the engine, and feel the brake servo give braking assistance. The pedal should sink. In my case, the pedal only sank a little. Turn the engine off and the pedal should stay where it is for 30secs (at least) because the brake servo is holding a vacuum and hence continuing to provide assistance. In my case, it pushed back immediately.

Test 2

Remove the vacuum line from the engine to servo and test that the one-way valves are working as expected. I have a hand operated vacuum pump with built in gauge to verify this, but you can do it just as easily by blowing through the valve. You should be able to pass air from servo side of the valve to inlet manifold but not vice-versa.

Test 3

Make sure the engine is actually generating a vacuum at idle. Disconnect the vacuum pipe from the inlet manifold and test that you can hear it sucking in air. Do not put your finger over the end!

Test 4

Use a vacuum pump & gauge to generate a vacuum into the servo & see if it holds. In my case, I was pumping like crazy and the vacuum bled down quicker than I could generate it.


After these tests, it was clear that my brake servo was not holding a vacuum even though my one-way valve was working and the engine was generating vacuum.

I bought the cheapest new servo I could find on ebay. Even though it was supposedly for an MGB, I found that the 4 bolts that hold it to the pedal box were wrongly spaced. I had to drill out the holes to 10mm to give some additional clearance. I suspect that this servo had either been mistakenly made with metric spacing, or it was actually from another vehicle that happened to use the same master cylinder but used metric throughout. Either way, it works well once fitted. My brakes are now very good indeed!