Brake servo operating too sharply

[Mckorkins]

I've got a standard Lockheed servo as per type 6/7 on my Moggie to which I've just fitted the Ford-type disc conversion kit. Yes, the brakes were a bit spongy for a couple of hundred miles or so, as warned they would be. To the problem: after first starting the engine, if I apply the brakes, the pedal immediately becomes stiff and comes UP rather than initially going down before resuming normal pressure. There is a vacuum being created and there appears to be no leaks. The result is that the servo kicks in sharply and does not give progressing braking. Any clues would be helpful. I am in France and are faced with a 400km trip to the ferry terminal in a couple of days. Help!

[geoberni]

Try looking at the MGA Guru site. There several pages about 'Power Boosters' (American site)... including:
http://www.mgaguru.com/mgtech/brakes/bt201l.htm
http://www.mgaguru.com/mgtech/brakes/bt201m.htm

[Mckorkins]

Thanks for the reply and links. Most helpful. I had already sussed out that blanking the vacuum pipe at the manifold would be a possibility, as I cannot effect a repair in France. I've already tried the brakes without engine power (coasting down a hill) and they work just fine. Perhaps a strong piece of duck tape over the manifold outlet will do the trick until I get home and will investigate the problem further then. Wish me luck.

[oliver90owner]

I've already tried the brakes without engine power (coasting down a hill) and they work just fine.

My initial thought was that the wrong grade of pads may have been fitted. All the cars I have driven have needed very heavy pedal pressure when the servo vacuum has been lost. Maybe your system is different?

[philthehill]

Blank the manifold hole with a proper blanking plug or disconnect the pipe at the servo end & insert a bolt into the vacuum pipe and secure with a jubilee clip.
A piece of duct tape is not sufficient to blank the servo and could cause a dangerous situation where the duct tape fails, and the brake servo operates causing the brake to lockup at the wrong moment.

[Mckorkins]

Good advice, although I've got no blanking plug with me. I've managed to find a self tapping screw with an inbuilt washer that I've managed to put into the pipe and covered it with the duck tape. There's very little chance of any air ingress now and I'll put a cable tie around the pipe as well.
It puzzles me that when the engine is started with a foot on the brake pedal, it should initially go down, but exactly the opposite is happening as the pedal is being forced UP! It seems that the vacuum created is acting on the input/supply side of the brake. When fitting the discs, one had to modify the master cylinder and remove the rubber non-return valve in order that the pads release when not under braking conditions. I find it hard to believe that it is a source of my woes though. A non-return valve wasn't fitted in the line between the manifold and servo initially. I just wonder whether it should have been, although it is used solely to retain the vacuum. Perhaps I'll fit one once the repair is effected
I'm sure one of you gurus will have an answer to my servo troubles - hopefully!

[Sleeper]

I have been told that to test the servo , press the brake pedal, start the car and the pedal should rise...

Many servos have the non-return valve in the servo connector..

John ;-)

[Mckorkins]

Every post I've read says the pedal goes down! Perhaps there's not too much of a problem if you're correct! Where did you get your info from?

[philthehill]

Press the pedal
Start the engine. the pedal should go down.
The servo comes into play when the engine is started so pulling the pedal down.
Confirmed by trying the above on my servo assisted brakes and yes, the pedal was pulled down.

[Mckorkins]

Yep, that's what I thought. So I do have a problem somewhere in the brake system. It'll give me something to scratch my head about when I get home - or if!

[Sleeper]

Since most vehicles use a vacuum system, the brake booster can be tested at your home. Observe the following 3 steps:

With the engine off, pump the brakes — about five or six times is sufficient. This depletes the stored vacuum.

Turn the engine on while pushing down lightly on the brake pedal. If your brake booster is working normally, the pedal will fall away a little, but then become firm.

If your brake booster is not working correctly, nothing will happen, or the brake pedal will push back against your foot once the engine starts. This could be a sign of a brake booster problem or an issue with the vacuum hose.

If you notice the brake pedal is difficult to push, higher than normal, and your vehicle takes longer to stop, have a mechanic inspect it to be safe on the road. If needed, the mechanic will replace your brake booster in a timely manner so you can safely drive your vehicle again.


Maybe I misread this?

John

[Mckorkins]

No you didn't misread this! It's exactly the problem I'm experiencing. But what is actually causing it mechanically? I do not want to have to replace the entire servo, which generally is in VGC. I can only think it is a problem with either the diaphragm or the plunger. A refurbish kit is available, so maybe I'll go for that, although one was fitted two or three years ago. A puzzle...