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Master brake cylinder replacement - 1976

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FlaMGB Tim S
Gainesville, FL, USA   USA
1976 MG MGB
I need to replace my master brake cylinder - according to the manual and looking at it in the car, it looks really straight forward - disconnect the three brake lines, and remove the 2 nuts on the brake booster, remove the old one M/C, hook up the brake lines to the new M/C, tighten the 2 nuts connecting the M/C to the brake booster, bleed the brakes. Am I missing anything? I read posts, and many talked about problems with the pedal box - but I assume those are for cars without the booster?

Also, do I need to put anything on the brake line connectors when I screw them back in to keep them from leaking? Or will they seal okay by just screwing them in?
And, should I bleed the M/C before putting it in?

thanks!

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ghnl Avatar
ghnl Silver Member Eric Russell
Mebane, NC, USA   USA
1961 MG MGA "Calvin"
Basically, that is it.

Do not put anything on the threads or ends of the brake lines. The seal is made when the fitting clamps the flared end of the brake line into the port on the master cylinder. Snug but no so tight you strip the threads or round over the flats. A proper flare nut wrench helps to minimize the chances of rounding off the flats.

It can help to 'bench bleed' the master cylinder but I've done it without. It will take a little longer and maybe take more brake fluid if not done as you'll be pushing air against the fluid already in the rest of the system.

Note that DOT 3 & DOT 4 brake fluid (the 'regular' stuff) will eat away at paint if spilled. Have a spray bottle of soapy water handy (an old Windex type spray bottle works well) to wash up any spilled brake fluid ASAP. DOT 5 (aka silicone) doesn't eat away at paint but shouldn't be mixed with DOT 3 or DOT 4.

Be sure to carefully road test the car in a safe area after completing the work. Brakes are fairly high up on the List of Important Things.



Eric Russell ~ Mebane, NC
1961 MGA #61, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100

FlaMGB Tim S
Gainesville, FL, USA   USA
1976 MG MGB
Thanks Eric, I really appreciate it. The soapy water bottle is a great idea.

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rrmgb Avatar
rrmgb Silver Member robert schau
Reston, VA., USA   USA
1973 MG MGB
1973 MG MGB
Put as many damp rags under your work area (where the fittings are) that you can find.
If you have the means, suck out as much fluid as you can before removing the fittings.
Dont forget the warning switch on the bottom of master. If it won't come out or breaks (plastic),
just plug the hole on the new master (short 3/8 fine thread and washer) or order a new switch.
The nut holding the master on the outboard side is tricky. A 1/4" drive universal 1/2" socket is your friend.
Bleed the master (3 fittings) in place (remember the rags?) before bleeding at wheels.
Good luck! Or are you finished already?



"It is a mistake to try to look too far ahead. The chain of destiny can only be grasped one link at a time."
Sir Winston Churchill
---------------------------------------------------------------
"It all starts in your mind's eye, then it goes to your heart
and finally to your very soul."
G.S.George PHD

ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
1972 MG MGB
If there are any shims/seals etc between the MC and the booster, reinstall those as they were on disassembly.

MG Logic Phil Pierce
Newtown, CT, USA   USA
Slide a large piece of plastic like a garbage bag between the m/c and the inner fender all the down below the bottom of the car to keep brake fluid from getting on any painted area. Keep the brake fluid off your skin (and eyes!).

BumbleB74 Avatar
BumbleB74 William Milholen
Tidewater, Tidewater VA, USA   USA
consider replacing the seal that goes between the master cylinder and the booster. You get that seal when buying the rebuild kit for the MC.

When you go to re-install, many find it easier to attach the lines with the master floating in the air a little bit as opposed to bolting it in place. Give that a try if you can't get the pesky lower brake line started!



1974-1/2 Roadster, "Bumble Bee", Corvette Yellow - in shambles, wire wheels
1976 Roadster, "Virus", Sandglow - "driver" condition (stock + 32/36 Weber DGEV, cast iron header, 25D distributor), bolt on wheels, ON the road!

FlaMGB Tim S
Gainesville, FL, USA   USA
1976 MG MGB
Thanks!! I really appreciate all the help. Great suggestions. I have ordered the MC, and I also ordered an eezibleed. Thanks about the switch - I forgot about until I saw it last night. Thanks about the bolt size.. I suspect the switch may break removing it. The MC looks pretty old. I couldn't find the leak and then noticed my inside wheel well looked a little wet and sure enough that's where the fluid was going. The one time I'm glad I haven't repainted the engine bay haha. But I will use the garbage bag idea! I didn't realize there may be shims/seals between the master and booster - I'll keep an eye out for them. Thanks for the socket suggestion - I'm sure I will use it. I should get everything by Saturday - so hopefully by Sunday I'll driving - and stopping haha. I love this place.......

rrmgb Avatar
rrmgb Silver Member robert schau
Reston, VA., USA   USA
1973 MG MGB
1973 MG MGB
If using the plastic bag....have a drip pan on the floor !



"It is a mistake to try to look too far ahead. The chain of destiny can only be grasped one link at a time."
Sir Winston Churchill
---------------------------------------------------------------
"It all starts in your mind's eye, then it goes to your heart
and finally to your very soul."
G.S.George PHD

BumbleB74 Avatar
BumbleB74 William Milholen
Tidewater, Tidewater VA, USA   USA
warm soapy water is your friend to clean up any spilt brake fluid...do it immediately......not later.

New switches can be bought.

Watch this video, in particular about the booster seal. The Sil-Glyde stuff is great!





1974-1/2 Roadster, "Bumble Bee", Corvette Yellow - in shambles, wire wheels
1976 Roadster, "Virus", Sandglow - "driver" condition (stock + 32/36 Weber DGEV, cast iron header, 25D distributor), bolt on wheels, ON the road!

tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
You might consider converting to DOT 5 while you are at it. Safe on paint, and it may be the last time you have to do anything to the brakes. I just converted my clutch as it is a new slave. Filled the master with brake cleaner and "bled" several times. Let it sit overnight and did it again. Now flushing with clean DOT 5 and see no contamination. The seals do absorb some fluid, as does the hose linings, I will flush again again a few month with fresh DOT 5. If concerned, front calipers have just a little O-ring in them to change for a full "rebuild" Rears are too cheap to rebuild. If hoses are more than 10 years old, change them anyway. You will then have the confidence of 100% fresh brakes.



Cogito ergo sum periculoso

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