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MG MGB Technical - brake pumping

Cannot get brakes on 80 MGB to work properly.
Have added new flex lines, new master cylinder and adjusted rear brakes. Have bled brakes several times even using Mityvac and still can't get a firm peddle.
I'm not losing any fluid. Help!
R W Cappello

Have you tried leaving the bleeders open and letting gravity do the work. I too have tried Mityvac with stubborn clutch slaves, and from the this BBS it was recommended to let gravity do the work. I was pleasantly surprised to see how effective it worked.

Shareef Hassan

With both my MGBs I find I have to bleed in two stages or I end up with a soft/long pedal. Firstly is to fill the system with an EeziBleed until no more air comes out, but that leaves the soft/long pedal. Then I have to get someone to stand on the pedal while I rapidly open and shut each front bleed nipple in turn, and this always gets an extra 'lump' of air out, and the pedal is fine after that. I don't see how leaving it drip would help mine, as it has been repiped and there are some quite big loops of pipe that will hold air. I think the high-pressure bleeding either blasts these pockets of air down the pipe, whereas the gentle flow of the EeziBleed doesn't, or possibly it is air bubble stuck to the side of the pipe that the pressure squeezes down and moves, and the rapid flow then shifts along. Or a combination of both.
Paul Hunt 2

Thanks Shareef,I'll give it a try but I'm not optimistic. Somewhere, somehow I think air is getting into the new master cylinder. this 80MGB has a dual brake system that is somehow connected to the clutch.

Roger. The MGB never used a master cylinder that was housed in a single unit and operated, having two bores, the brakes and the clutch.

On the 1980 MGB you have a dual circuit master cylinder with a brake booster (servo) before the master cylinder. The brakes on these cars are not quite as firm as the earlier model cars, perhaps due to the power brake system. They are, however, capable of being tuned to give a good, reasonably firm, pedal pressure. Follow Paul's advice if you have an Ezi-Bleed. If not, bleed them in the traditional manner having someone depress the pedal to the floor, hold it there, open the bleed nipple, close the bleed nipple, and repeat until you see no air in the expelled fluid. Keep topping up the MC reservoir as necessary.

A note on the order of bleeding the brakes. Everyone knows you bleed the brake furthest from the MC first, followed by the next furtherest, etc. What most people do not realize is that, unlike an American made car, where the right rear brake is furthest, on the MG the furthest brake is the left rear and it is the first one to bleed, followed by the right rear, the right front caliper, then, the left front caliper. This is due to the car having been designed as a right hand drive and the system adapted for left hand drive. Trace the lines and you will see what I am writing of.

Les Bengtson

A question has been nagging me as I get ready to bleed my now complete brake system. I 'plan' to use my Mityvac first, then use some form of the traditional method. But, gravity bleeding intrigues me if I get no joy from my initial methods.
I've read past reference to blocking the pedal down overnight, but it seems to me that the passage between the lines and the reservoir will be open with the pedal up.

Shareef, when using gravity, did you leave the pedal up, or block it down?

Thanks, Joe
Joe Ullman

Blocking the pedal down is done for the clutch system, not the brakes.

There's a loop of clutch line near the master cylinder that's higher than the master and traps air bubbles. Because it's so near the master, it can be hard to bleed out the traditional way. Keeping the pedal down overnight keeps a volume of fluid in the slave cylinder while the air rises into that loop. Letting the pedal up causes that fluid in the slave to rush back into the reservoir, and it seems to carry that trapped air with it.
Matt Kulka

A problem with the Mity-vac is that it can suck air past seals, especially the wheel cylinders - they are designed to seal against pressure, not vacuum - you can bleed forever with no gain.

I pump the brakes a few times to get fluid in the system, maybe bleed a time or so at each cylinder with the pedal blocked down. I work alone. Then I open the farthest cylinder and let it drip, keeping the master full. When it is a steady drip, close it and open the next one. On a split system you can do two at a time, one front, one rear. The result is always a rock hard pedal with maybe a half inch travel if you lock up the adjusters first, which I recommend.

If this doesn't work, then there's a loop in the pipes as Paul said - fix the pipes!
FR Millmore

Thanks guys, that clarifys things a bit. FR, I'll try it that way. From that I'm getting that the pedal is up during the gravity feed part, and that I shouldn't have to block the pedal if I'm using a method to prevent air from coming back in during pedal release. Joe
Joe Ullman

If the pedal is blocked down, the ports in the master are closed - no fluid will flow. When you take a line loose, block the pedal down to keep the system from draining - saves a lot of fluid and mess - and bleeding!
FR Millmore

The back brakes are the main problem. A real pig.
Last time I used a 50ml syringe and some tubing.
I dissconnected at the master and syringed 50ml though, then reconnected. I then pedal pumped any small air bubbles though.
Next time I'm going to try attaching a vacuum cleaner to a drop out bottle. Thats any old largish container (sealed) with two tubes in it. One goes to the bottom and the other the top only. Vacuum goes on the top outlet and the brake lines are connected to the other end of the bottom outlet. Any "destroy your vacuum cleaner fluids" are left in the bottle while air gets sucked out.

I want to thank all you guys who have responded to my original request for help. I've tried several of your suggestions and still do not have a firm peddle. There is improvement though. I still have a long peddle which is disconserting.
Les, you mentioned the clutch and brake cylinders are not connected but there is a third line coming out of the brake master cylinder and heads toward the clutch master.
Do you know where it goes and if I could be getting air through that line?
Roger Cappello

If the pedal travel is on the long side but firm then I'd suggest checking the adjustment of the rear brake shoes. The rear brakes can take up quite a bit of travel if they're not properly adjusted.
Bill Young

Per Bill's suggestion,you will note that I recommended that you lock up the rear adjusters before bleeding. That eliminates one source of movement. Another is caliper/disc flex from a stuck piston - you can usually see it if you have someone else lean on the brake while you watch the caliper carefully. One stuck piston can eat all your travel. With both of these taken care of, there is nothing to move, so any travel is air.

No lines connecting the clutch and brake circuits - follow the pipes to see where they go!
FR Millmore

roger, the dual circuit M/C used on the late model MGB'S had one line going to both rear brakes and a separate line to each caliper on the front. you are seeing the line going to the right caliper. i have not seen a logical reason to use 2 front lines from the M/C, rather than a Tee. it makes this M/C very costly and difficult to swap for an A/M brand such as Wilwood or Tilton. i have used the gravity flow bleed methods successfully, but usually i conclude with the pump,hold and bleed sequence with Dot 4 fluid.
kelly stevenson

This thread was discussed between 19/04/2007 and 27/04/2007

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