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MG MGB Technical - Bleedin Brakes & master Cyl

Hi folks

having a tough time bleeding the brakes ( UK- 77 BGT) and am sure its the master cylinder. I'm on my second set of seals and am still not convinced I've got them the correct way round.

Enclosed is picture showing orientation I hope you can see.

The first part of the piston to be inserted into the bore has the seals pointing the correct way, but are they the correct ones/order ? ( Thick & Thin)
After the second spring is inserted the piston has 2 different thickness seals, are these in the correct order and pointing the correct way ?
I've watched JT youtube loads and its not shown clearly..

All I know is at very the bottom of the brake travel there's a ppfffft for about 1/2" to 3/4". The front brake calipers get nuffin and a mere dribble to the rear when bleeding.

Should I bite the bullit and buy a MGOC/Moss aftermarket MC?

Paul Liles


According to the Haynes, for the piston that goes in the bore last, the cup seals both face the same direction. That direction would have the larger OD of the seals going into the bore first.

It is kind of hard to tell from the pic, but it seems that the thicker seal would be the last one of the four to enter the bore on assembly.

This all assumes you have the MC with the rectangular reservoir, as would be on a USA 77. If you have the round reservoir, ignore what I said.

C R Huff

Hi, Paul,

what else have you done to the brakes? Is the master cylinder the only thing you've touched or are there other parts which have been dismantled? There are other things which can cause mysterious problems when it comes to bleeding.

I would have thought that bleeding the brakes would do more than you describe even if the seals were on incorrectly.


Mike Standring

Loks as if the seals are pointing the right way . as for which is which:-

My MG workshop manual says
17 fit a shim washer to the primary and secondary pistons.

18 select the 2 identical seals and fit with the lip pointing away from the shim washer.

19 Fit the thinner of the 2 remaining seals to the secondary piston with it's lip towards the primary spring seat.

20 Fit the second seal to the primary piston with its lip facing towards the piston seal

21 compare the return springs, the secondary spring is the shorter.

I can't see if you have shim washers on your pistons.


V Todman

If you have the split reservoir, ensure that both halves are full. I kept seeing a 'full' reservoir only to find the other one was empty. Can't remember which half does which circuit, but they are separate & won't top each other up even though they share a filler.
P M Gregory

It looks like the last seal is back to front. Part No 20 in the diagram pictures above.

Thanks for the input guys.
To answer the questions :-
1/. The whole of the copper pipes have been renewed
2/. Front calipers rebuilt with new pistons and seals
3/. New rear Rear cylinders and brake springs, new adjusters ( handbrake works fine)
4/. The M/C cylinder piston/s has the shims in as they should

The only thing not changed are the bleed nips themselves ( different types from front to back) and the front rubber brake hoses.

The copper pipe is not constricted anywhere along its length and the rear 3 way union works as it should.

After looking some more, the piston which is inserted first does not seem to spring back when depressed by the second piston, i.e the spring is too weak.

I've heard a lot about the quality of rubber goods being sub-standard...could the seals be too large or too tight ?

Next time time is available and family members willing I will revert the last seal as mentioned by Roadwarrior.

What I can't understand is why is there no difinitive diagram anywhere. Everyone I've seen ( even JT vid) has been vague to say the least.

Once again thanks to everyone

Paul Liles

Have had another look at your image and now agree with Roadwarrior that the seal on the right is back to front.
V Todman

Dear Paul,

The classic mistake is to put the callipers on with the bleed nipple at the bottom. The result is brakes which are impossible to bleed and is normally only found after the second new master cylinder has been installed...

Easy mistake to make but easy mistake to check for!


Mike Standring

Most of the original Lockheed and Girling rubber parts have the part number moulded into them. Check, if you still have the old seals.
Also worth a look is;
I think that's right, but google if not. They have parts lists and exploded drawings of virtually everything from 1938 on. He's a one man band with a depth of knowledge he's willing to pass on, and a large stock of OE parts.
Allan Reeling

I have just checked out your recommendation
What a GREAT website,I recommend we all add
to our favourites bar

wm griffiths

This thread was discussed between 07/07/2014 and 09/07/2014

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