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MG MGB Technical - Clutch Master

Unable to get the clutch system to bleed after rebuilding the slave and replacing the master, I disassembled the master to see if there was a problem and found it’s structured differently than I imagined.

The piston assembly that is pushed by the clutch pedal through the bore has two seals. I expected the primary seal (the deeper one) to be behind the opening to the reservoir until the pedal was depressed at which point it would pass that opening thus sealing and force the fluid down the line. I expected the function of the secondary seal to prevent the fluid from draining out the back of the bore after the primary seal passed the opening to the reservoir.

What I found is that the primary seal is about a half an inch past the opening to the reservoir when the unit is assembled. Removing the circlip and pulling the piston assembly so it protrudes from the bore a half an inch, I could blow into the reservoir and air would exit the outlet connection (the pipe to the slave cylinder). I expected air to exit the outlet connection once the piston assembly was completely installed, as that is the expected path of the fluid from the reservoir during the bleeding process. Once the piston assembly is completely within the bore and the circlip installed the opening to the reservoir feeds fluid into the center chamber between the two seals. So, how can the system be bled if the primary seal never goes back far enough to pick up new fluid from the reservoir?

With the cap on the master cylinder reservoir the plunger can be pushed in to force air out the outlet at the end and covering that outlet will show a vacuum as the plunger is returned under the pressure of the internal spring. Just before the piston assembly completely returns to its maximum position, the vacuum is lost. Now I expected that very same thing to occur when I thought the primary seal would move back past the opening into the reservoir, but now I know better. Does the bore have a taper that looses the seal as the primary seal gats so far back or is my unit defective? A taper would allow fluid to go past the seal and be captured as the seal mover forward.

I’ve bled both the brakes and clutch systems many times on MGBs, so I don’t know why I’m having so much trouble this time.
George Champion

Nobody has answered here yet, so I throw in a few sentences to maybe stir things up.

What type of clutch master do you have?

Early type - with the "cup" primary seal? ...or...

Later type - with the "donut" type primary seal.

The later type also has an external groove on the cylinder body to identify it.

I do not believe the pistons and cylinders are interchangable between the two types,
but I could be totally wrong on this.

Whenever I run into a renewed clutch m/c that is stubborn to initially bleed - I use a Mity-Vac on the
slave cylinder bleed screw to forcibly draw fluid through the reservoir, into the system. For stubborn
air bubbles, I will sometimes reverse-bleed the system and pump fluid through through the slave
bleed screw back up into the system. It'll then take a few strokes of the clutch pedal to pressurize
the hydraulic system.

Although the primary seal does not draw past the reservoir feed hole, fluid is allowed into the system
as the seal is a "one way" design. That's why you have to install the seals facing in the correct
direction. Also, on both types, there's a thin steel curved washer behind the primarly seal that acts
as a feed valve as well as a system pressure regulator.

When you let up on the clutch pedal, the curve in the washer exposes the feed ports on the piston
and a certain amount of fluid is drawn past the seal (through suction, assisted by the piston return
spring).

At rest, a little fluid is allowed to leak past the washer slightly and flow back into the reservoir - but a
certain amount of pressure is always maintained in the system. Have you ever noticed how the clutch
slave cylinder is always pressing on the clutch arm - which maintains contact between the throwout
bearing and the clutch. This is how the clutch system on a "B" is a self-adjusting setup - but it also
pushes the crank forward against the thrust bearings at all times.

When you depress the clutch pedal, the seal presses against the washer (flattening it)- which seals
the feed ports on the piston- the primary seal then expands slightly as presses against the cylinder
wall - and retains fluid and pressure.
Daniel Wong

Try draining and refilling the system using a gunsons Eezi-Bleed on very low pressure on the *slave* nipple. Did mine this way and it didn't need any further bleeding at all. You are looking for about 1/2" travel at the slave piston.
Paul Hunt

Thanks for the replies.

To answer Daniel’s questions, my ’77 originally had a marked master cylinder with the donut primary seal and the rebuilt replacement was unmarked with the cup primary seal. From what I gather from the Moss catalog, the housings are the same except for the marking that indicates which type of piston unit is within. Studying the seals and other parts seams to confirm this, but I didn’t do any real measurements.

I did try using a Mity-Vac, but it began to leak. I would also like to try the Eezi-Bleed from below. I just couldn’t figure how it could push the fluid past the primary seal when flow from that side would expand the seal to seal off the reservoir.
George Champion

When the clutch pedal is fully released the port from the reservoir to the cylinder is open. It is only closed off once the piston has started to move.
Paul Hunt

Now I’m going to have to go back and measure it for accuracy, but when the clutch pedal is fully released, an internal spring pushes the piston unit back against a circlip. The distance from the circlip to the port opening into the reservoir is about an inch and a half whereas the full length of the piston unit from where it rests against the circlip to the forward end of the primary seal is about two inches. That’s what got me to ask the question in the first place on how the fluid from the reservoir gets past the primary seal when the seal doesn’t come within a half inch of the port. This also gets me thinking that the internal spring seems so strong that it would flatten the curved washer under the primary seal preventing it from functioning as a check valve.
George Champion

This thread was discussed between 25/02/2002 and 05/03/2002

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