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MG MGB Technical - Calling Master Cylinder Gurus??

Please refer to the attached image...

Background: I had my 69B TANDEM master cylinder re-sleved recently. (I know, I probabally should have done an exchange but this only cost me a bottle of wine... I may yet have to do the exchange or have a professional rebuild$$$$??!) Today I tried to charge and bleed my new-build brake system (incl lines) but my master cylinder refused to pump any fluid.

What I would like to know, in order to try and fault find, is what is SUPPOSED to happen inside the MCyl. when the pedal is depressed?

Is the secondary piston (red) supposed to move??

In my case, the Primary Piston (17 shaded) moves into the cyl and then springs back. Some air pressure can be felt from outlet "A" but nothing at all from Outlet "B". Conversly (and probabally why no fluid was sent down the lines) when the piston is depressed if I then place my finger over outlet 'A' Suction is felt... ie the fluid must be getting pushed then sucked back?? The secondary piston is almost certainly not moving. By the way, The holes which drain the reservoir into the Mcyl body have been dreiied through the new sleeve!

I would have thought that both outlets needed to be 'pumping'??

I am wondering if I have made a fundamental assembly error? The image in the middle shows the order in which the bits are assembled and as far as i can tell, is correct.

Also, the Pressure Fail switch has been turned clear of the shuttle as per the "Haynes" manual Bleeding instructions

Any ideas?

Innes Bint


may be there has something gone wrong during reassambly. Both the pistons are 'coupled' together by a clip and a pin. No. 9 and No. 10 in the image you added.

Have these two items gone into position again? Seems the smaller piston is not operated from the pedal due to a missing connection.

Hope this helps



I have been looking at this project as I have a 68 BGT in need of a rebuild or replacement tandem unit as well. From what I can noodle out the secondary piston should be driven down the bore by fluid pressure acting on the secondary cup (#7)only, and the piston link is only to pull the secondary back into position when the pedal is released. In the middle image of the photo you have attached does this show the seal position after the rebuild or just after disassembly? From what you describe I wonder if is it possible that not all of the holes between the reservoir and piston bore have been fully opened? My hat is off to you by the way for attempting this project.

Cliff Maddox

Innes, be sure that the brake light switch isn't screwed in too far so that the pedal isn't allowed to return fully. If that happens, the main cups can't "pick up" fluid from the reservoir and force it out of the ports. Common error on a complete installation. BTDT

Peter C.
Peter Caldwell

Thanks guys..

Iv been looking into my problem a bit more... last night I actually completely removed and disasembeled the unit AGAIN??! and checked for obvious errors

This morning I phoned the local 'brake guy' who simply asked "did you bleed the master cyl first?"

I had never considered it!! I went straight out to the shed and tried bleeding each outlet (finger over the hole method) with the MC in the vice. It charged fine and worked as it should!!!? I then had to somehow return it to its mounts full of corrosive fluid?! By the way the brake man talked, I get the feeling that most MCs have a bleed screw or something to get them started?

somehow i got it back in but then had to take out my makeshift bungs and re attach the lines. during this process, fluid inevitably leaked out(thankfully onto the dozens of rags protecting my new paint!) and i was left in the same position as yesterday!! curse it. i have since got the front lines charged and bled but try what i may, the rear circuit will not start pumping!

i am about to try and build a pressurised reservoir to connect to the rear bleed screws and reverse fill the circuit... or at least try!

Pete.. the switch and cover are completely removed but thanks for the tip.

Cliff.. the only issue with dismanteling the unit is the nylon bush which often gets stuck and has to be pulled out via screwing in some long self tapping screws and pulling with pliers. the other is the inner-most circlip. i got mine out with some nails welded / soldered to a set of old pliers. other than that, i cant see aproblem with it. lots of people would sooner replace or exchange but i try to repair/restore my old stuff and save a buck or two. (plus i kind of like pulling things to pieces! the seal kit contains a nylon bush so unless the bore is seriously pitted, it will cost about $15 rather than $250
to answer your Q, the photo shows the post resto state.

Innes Bint

Cliff is correct that the primary piston uses fluid to push the secondary piston. Until the space between fills with fluid you will get pressure only from the primary piston outlet. If you haven't already checked, make sure that the pistons return far enough to uncover the holes.
I don't know what nethod you are using to bleed your brakes, I have always done the following and it has never failed. Open the bleeder at the wheel and have someone else push down the pedal, when the pedal bottoms, tighten the bleeder before the pedal is released. Repete many times. Closing the bleeder before releasing the pedal prevents the fluid that has been pushed into the line from being sucked back to the master when the pedal is released. To get the last few little bubbles out put pressure on the pedal and then crack the bleeder, the sudden relaese of pressure will help carry out the last few small bubbles.
John H

When I have rebuilt one and put in on the car, I bleed the MS first. I loosen the lines, have friend push the brake down and hold while I tighten the line at the MS. Keep this up till fluid is coming out under pressure. Then bleed the rest of the system.
It is messy, but works for me.

Good Luck


i tried 'reverse' bleeding via forcing fluid up through the wheel cylinders but it would not go in. (i had made a container which i could pressurise with a bike pump which would force the fluid up and out)

after much head scratching, i tried pulling the MCs piston and 'volia', it returned an extra quarter inch. this must have been enough to uncover the reservoir holes (as John H suggests) and by repeating this method inbetween pedal strokes and cracking the bleed screw (normal method) brake pressure was achieved. woohoo

for reference...

1. New Master Cylinders (tandem type) need to be primed/bled PRIOR to attempting bleeding.
2. easily done on bench but the reinstallation and connection is difficult as fluid seeps out losing 'prime'
3. piston may have to be 'hand returned' to get the fist few strokes working properly.
4. bleed via regular method
Innes Bint

The best way I've found to bleed the Master cyl is in a vice with brake lines attached and bent in a "U" to discharge into the resevoirs below the fluid surface so you can see the bubbles, when the bubbles stop the M/C is bled. Don't remove the bleeder lines until you are ready to instal the lines on from the car. Ric

good thinking ric.

i have just realised why it seems more difficult for me to connect up the lines whilst not losing fluid(and my mind)... those of us who have RHD cars have the MC outlets squeezed up close to the RH guard.. it would certainly be a lot easier having the cylinder over on the other side and the outlets exposed!
Innes Bint

This thread was discussed between 22/11/2007 and 25/11/2007

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