Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Bench Bleed New Master Cylinder?

I finally broke down and ponied up the bucks for a new master cylinder. I want to get my brake problems behind me, so I checked the archives. The archives recommend that you bench bleed a new master cylinder. How is this done, and why?
Neil Glick

Neil. It verifies the operation of the master cylinder and allows you to get the air out of it prior to installing it.

Danny Wong has a tech article on how to bench bleed the master cylinder in the MG section of my website, which might be of use to you.

Les Bengtson

Be carefull installing the MC if you bench bleed it. Since it will be full of fluid it will leak out the line connections if you don't plug them. If you are using standard brake fluid the leaking during install can eat your paint.
John H

A normal MC bench bleed kit will contain 2 fittings to screw into the line fittings, and two short length of hose that go from the fittings back into the MC reservoir. You'd fill the MC while clamped in a vice, and pump fluid out the fittings, through the lines, and back into the MC reservoir until no more air passes through.

I'd install the MC and fill it, then pump the brake pedal about 100 times (no exaggeration) before bleeding the system. Going this route does the same thing as bench bleeding without as big a risk of damaging paint. The air will either be pushed into the lines where it will be bled out, or air bubbles will be pushed back into the fluid reservoir.
Jeff Schlemmer

Very messy, never bothered, never had any problems. The only benefit can be Les's first statement - that of verifying it works before you fit it. And if we could be sure that new parts worked properly on receipt you wouldn't even need to do that.
Paul Hunt 2


the dual line MC without servo (LHD export version 1971 to 1976) is a pig when it comes to bleeding, not comparable with the dual line servo assisted one ore the single line MC's.


Having benchbled 100s of MCs useing the system that Jeff S first described without any problems,I would suggest a bench bleed,sometimes the amount of air in the rest of the system is almost nil. Starting with a dry MC can take a lot of time,and you had better hope there is no problem with the MC as you might be pumping the air out for a long time befor you giveup and return an airbound good MC RIC

Can't see the point in bench bleeding a master cylinder. Why not just bleed it out through the right hand front caliper which is a pretty short line anyway.
Iain MacKintosh


the non servo dual line MC is of a different layout and makes use of two pistons with different diam. that are fitted inside and a lot of other details are different from the single line system or the servo assiste one as used on all late RB MGB's. This type of MC was used on LHD export cars only and, as long as no presurized bleeding system is available, bench bleeding this type of MC is the fastes and securest way.


Wouldn't you just open one bleed valve in each circuit with a dual master? Non-servo or integral servo?
Paul Hunt 2

Meanwhile, i allways use compressed air for bleeding hydraulic systems but remember the trouble this type of MC made befor i switched to forced bleeding. All the other hydraulic cylinders used on the B can be bleeded much quicker than this one. I considre the different diam of the pistons and a poor layout of the check valves beneth the reservoir are the outstandig reasons for the difficult bleeding procedure with this design of MC.


This thread was discussed between 13/04/2008 and 16/04/2008

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.