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MG TD TF 1500 - Master Cylinder Piston(s)

Two master cylinders arrived with my TF. I selected the best bits and rebuilt one good one using a new seal kit. I've spent a day (and a small fortune on hydraulic fluid) trying to bleed the system. The issue is, pump the peadal twice and it is rock solid. Leave it for a couple of seconds and I have to repump again. I've bled the system over and over again and there is absolutely no air in it. In desperation I have removed the master cylinder and dismantled it. I now note the two pistons I have differ. If you look at the end where the small holes are one if flat and the other slightly convex. The washer in the kit I purchased is flat. The manual mentions a wavy washer and that it essential this is fitted. Could this be preventing fluid entering, in front of the piston, hence the need to pump the pedal twice. Help will be much appreciated - my clothes are soaked in brake fluid and my hands feel awfull!


Jan T
J Targosz

Taken everything apart again. Photo shows the differing pistons anf the shim which came in my overhaul kit.

Jan T

J Targosz


Did you check the Compensating hole was clear?
Possibly take/cut a piece of wire out of a wire brush and jam it in the end of a 6" bit of 1/4" dowel as a tool. Carefully - make sure the hole is clear.
You should be able to see it through the filler opening.
Also check all the other rubber washers are in and seated correctly.
Just a thought.

R D Jones


That washer is not flat...note that it is slightly bent in the middle...that is why it's called a "wavy" washer.

They're used in most master's a view of them in the MGA kit:

Also, you may need to shorten the length of the piston slightly so the rubber will clear the compensating hole when the piston is fully contracted.

Gene Gillam

In looking at the photo it looks like the end of one piston is convex. Does that design eliminate the need for the wavy washer? All the pistons I have worked with with MGTDs and Morris Minors, are flat on the end like the one on the left.
John Quilter (TD8986)

Hello Jan. I think that you are looking at the problem the wrong way. If you need to pump the pedal twice to get the brakes solid the the wheel cylinders are not adjusted correctly. You should never need to pump the brakes twice in normal operation (dangerous). Re-adjust the brakes, only backing off the adjuster enough to get the drum to turn, one or at most two clicks. If you need to pump twice you are having to move the pistons an excessive distance to get the brakes to lock up. If you have put on new brake shoes were they arched to the drums? If not then the radius doesn't fit right and you will need either to have them arched or drive the car to bed them in.

R Taras

Thanks to all and especially Butch. Reassembled the master cylinder using the flat piston and adjusted brake shoes - I had let them slack to get the drums on and hey presto a nice firm pedal.

Jan T
J Targosz

This thread was discussed between 14/09/2013 and 15/09/2013

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