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MG Midget and Sprite Technical - Mongrel Master Cylinder

My MkIV Sprite came with a brake pedal travelled a bit too far and with too spongy a feel for my liking.

I adjusted as per the handbook, flushed through with lots of fresh fluid and bled several times with some improvement but not enough so I decided to re-seal the master cylinder and fit braided hoses.

The hoses went on fine but I came a bit unstuck with the master cylinder. It's a single circuit 'upright tin can' style which I'm sure is original and correct for the model. When I stripped it I found that the main piston appears to be a later style, matching the Haynes diagrams for the remote reservoir type. The piston has an annular primary seal that fits around a protrusion on the front of the piston.

Having cleaned and checked everything (clean bore and piston, no scoring)and put it all back together, the brakes feel better than before but I still think there's room for improvement.

The questions are:
Is this mongrel master cylinder an original from one of BMC's change-over periods or has a PO been monkeying about?

Does it matter?

Do all single circuit MCs have the same bore and stroke dimensions?

I have very good documentation covering the '90s recording that new MC seals were fitted in 1992 at 70k miles but there are no notes about pistons or complete MCs being changed. The current seals appear to have done about 30k miles.

I'm hoping the consensus is that the set up is OK as I'd much prefer to pay for a new set of seals rather than for a complete replacement master cylinder!
Colin Mee

I don't know what Haynes shows (never trust Haynes myself) But there are two versions of what Moss call the Type 1 m/cyl.
http://www.moss-europe.co.uk/Shop/ViewProducts.aspx?PlateIndexID=1210
You need the right kit for the cyl that you have.
David Smith

Thanks David

Those diagrams have helped a lot. It looks as though mine is the GMC 151. I just need to confirm that I've got a groovy body and then I'll get a set of the second sort ordered.

Strange the Moss and MGOC diagrams don't show the piston. Haynes and my BL MkIII manual do but only with flat ended pistons for the tin can reservoir types. The 'tipped' piston is shown in Haynes, but only on the remote reservoir type.
Colin Mee

This thread was discussed on 18/01/2013

MG Midget and Sprite Technical index

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