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MG MGA - servo check valve

The fitting instructions for the Lockheed servo on an MGA say that a U bend must be formed in the vacuum pipe and that for high performance cars a check valve is needed. Have members with servo's fitted this check valve and if necessary where do you obtain it? Can the U bend go anywhere in the length of the vacuum pipe and whats its size?
J H Cole

The purpose of the check valve is to maintain vacuum at the servo when the engine vacuum is low. When under full throttle acceleration the vac is lower from the engine. The vacuum in the boost servo remains high for the first brake application if a check valve is used. Inline check valves that just go in the hose are available for most Toyotas for about $30.00
R J Brown

Moss has got them. They are required when installing the Moss Supercharger Kit into a car with a booster.

Rich
Rich McKIe

"U bend" is a makeshift liquid trap simlare to a sewer drain trap.. I'd gues it's a nod to the MGA's sub-optimal crankcase breathing and/or siamesed intakes. mounting the servo below the intake would compound the problem.

Reversion forces liquid out the tube. The "U" acts holds the liquid. To work, the outlet (towards the servo) should be higher than the inlet. Both (or, at a minimum, the outlet) should be above the intake manifold.

When you apply the brakes, the servo "dumps vacuum." Sucking the new air out should help control the level.

Make sense?
Bill

BTW- you may want to fit a vacuum reservoir- hot rod shops sell them for 25 bucks or you could pick one up at a junk yard for less. This installs between the check valve and servo.

Concerning the check valve, I wouldn't install a servo without one. It may save your garage door some day. Without one, the servo won't work at low speed until your motor warms up enough to idle smoothly. On the road, you may be OK since the throttle is usually closed when you tap the binders- unless you heel and toe.
Bill Eastman

Gentlemen, perhaps you can clear something up for me also. I always thought the check valve was to prevent an intake tract backfire from wrecking the diaphragm in the booster, it wouldn't allow any positive pressure to pass the valve, just vacuum. I can see from your description how the trap in the line would function, thanks.
Bill Young

The check valves only purpose is to enable power brakes to work during low engine vacuum or if the motor is off. Typicaly the brakes will work for two to three applications with the engine not running. Every car built with power brakes has this check valve. Most of them now a days are located at the end of the vacuum hose where it enters the brake vacuum diaphram chamber. Many older cars in the fifties had a remote vacuum tank as well. Without the check valve if you accelerate hard and than try to brake you will find all the boost is gone and life gets very exciting until the vacuum builds up again
John H

When I fitted my servo I rang Lockheed Technical and asked if the non-return valve was necessary on the MGA and they said it was not. My servo works well.
Mike Illingworth

Ahh, So, Grasshopper has seen the error of his ways. Thanks for the clarification. It makes sense now, I hadn't thought of the situations mentioned like the engine dying or rapid acceleration.
Bill Young

This thread was discussed between 05/03/2007 and 09/03/2007

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This thread is from the archive. The Live MG MGA BBS is active now.