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MG MGB Technical - 72 MGB Brake Servo Bleeding???

I have a UK Model 1972 mgb roadster with single circuit brakes and remote servo. (Chassis # GHN5 297853-G) The servo looks like a factory fitment so am assuming it was an option. I have just rebuilt the master cylinder as it was leaking through the front past the pushrod. I am having a problem bleeding the system and have been told by a local mg specialist that I have to bleed the servo, however I cannot find a bleed nipple on or near the servo. I have bled the system several times and am not seeing any air bubbles coming out of any of the nipples at the brakes. I have a good pedal after a couple of pumps on the pedal but I have to pump it up again if I take my foot off of the pedal. The brake pedal was always firm before I rebuilt the master cylinder (until it leaked of course. I cannot see any leaks anywhere nor do I have fluid leaking into the manifold as far as I am aware. Any suggestions on how to bleed the servo? I have been considering disconnecting the servo from the system as an alternative to trying to get the air out. I have read somewhere that the servo gives minimal assistance anyway. Is this a safe and feasable thing to do?
W R Jennings

This may seem far out to some, but this is the way we bled aircraft brakes from the ground up.

Run a hose (ie 1/4 inch)from the bleeder to an old pump oil can. Open the bleeder and pump the oil from the brakes into the master cyl then close the bleeder valve. Do this from all bleeders, should work.

Works for me, and I do it by myself, cleaner too.
James Huggins

If you remove the servo air filter, underneath you'll find a tiny piston which could be removed temporarily in order to let air escape. Having said that I've never found it necessary to anything other use my Eezibleed.
Miles Banister

I think the trick here is to have the servo at an angle. Raise the end that the hydraulic pipe is connected to and bleed as normal. This should do the trick.

Colin M...........
C E Muir

You don't have to separately bleed the servo. Both my cars have the remote servo and I have the same problem bleeding them, 'normal' bleeding with either the pedal or an EeziBleed leaves them with a long pedal that pumps up with rapid strokes, but release it for a few seconds and it is long again. I have found that after the normal bleeding I have to get someone to press down as hard as they can on the pedal while I rapidly open and close each caliper nipple in turn, which always gets an extra 'lump' of air out. Whether the high pressure is needed to compress and move the air bubbles off the walls, or whether it is simply the resultant very high flow as the nipple is opened I don't know, but it works.
Paul Hunt

I actually use that method after normal bleeding as it is the only way to finish a Harley I have. However I am on my third litre of fresh fluid now in bleeding and still not seeing an improvement in the pedal. I have no option but to keep at it though. I havn't been able to source an eezibleed here in Oz so am using the normal method first.
W R Jennings

It sounds as though you're no stranger to bleeding, but here's a thought. On re-reading your initial post I see that you've rebuilt the master cylinder. After I did mine I put several litres of fluid through and still couldn't get a firm pedal. In the end I disconnected the pipe from m/c to servo (can't recall now which end but probably at the servo) and used tubing to recirculate it back to the m/c reservoir. The idea was to find out where the air was without wasting gallons of fluid. Anyway, I noticed tiny bubbles in the tubing so pumped away for several minutes until it was clear. Then when I reconnected the pipes and bled the calipers again it firmed up. While we're on the subject of sucking eggs - you have adjusted the rears first?
Miles Banister

And sometimes master cylinders just don't take to rebuilding and never work as you would expect. If all else fails, you may need to get a new master cylinder.

Wayne Pearson

Drive the car very CAREFULLY to someplace where it can be pressure bled. Barrie E
Barrie Egerton

I have just refitted a rebuilt servo in my '73 GTV8. The advice from the fellow who rebuilt the servo was to pump the brakes hard and fast for several minutes before bleeding. The rationale being that it will cause the air to mix into a foam and pump through the servo.

It worked, the brakes are rock solid, now just need to sort the overdrive issue!
David (Oxfordshire)
David JM

I did adjust the rear brakes before starting this bleeding marathon (pun intended), in fact I'm in the habit of doing that every time I have the car up on stands. I think the M/C rebuild was successful as I can get a hard pedal after two or three pumps. However, it goes back to the floor once I release it. This tells me it is an air issue not a M/C issue. Is my thinking right in this???
W R Jennings

Well, everything you say points to the presence of air rather than a bad m/c. Could you rig up clamps on the three flexi hoses and see what happens? If the pedal is hard then there must be air - or excessive slave movement - in one or other of the pipes and releasing one clamp at a time would indicate which. If pedal is still soft then it must be either in the servo - or m/c. If so try taking out the tiny servo piston and give the servo a gentle bleed. If it is the servo - can you raise the right hand side of the car enough to get a rise on the pipe and try leaving it overnight to see if the air finds its own way back to the m/c ?
Miles Banister

The clamps on the hoses will only isolate relatively small sections of the braking system. With an intractable problem like this I'd be inclined to disconnect and plug the lines at various points *before* the servo, and at the splitter, to isolate more. I take it you have the caliper hoses at the bottom and the bleeders at the top, relatively speaking? Also try bleeding the rears with both pistons wired together i.e. back to back, and not simply pushed back as far as the shoes will do. If there is something wrong with the handbrake levers this could allow the rear cylinder pistons to be partially out, creating a space between them, which may not be fully bled.
Paul Hunt

Miles, I will try jacking up the right side of car only as you suggest but am unsure how to take the servo piston out or how to bleed it?
Paul, I will try wiring the pistons in the rears together as any space there makes sense as a possible place where air can get stuck.
Thanks for the advice I will consider anything before I hand this over to a professional.
W R Jennings

I've only removed the piston once - several years ago when I thought it might be sticking. I recall removing the whole of the white plastic air filter assembly and seeing it nestling just below. I didn't need to bleed the whole system afterwards, just put a few drops of fluid in the cylinder before replacing the piston. Hopefully if you have air trapped there then that would fix it. Good luck, and keep us posted.

I was given this tip from a very old car mechanic a few years ago. If you have bled the brakes but you still do not have a hard brake pedal, do the following: remove the brake reservoir cap, pump the brake pedal a few times and whilst the pedal is still down wedge a piece of wood from the brake pedal to the drivers seat, thus forcing the brake pedal down, and leave it like that overnight. The theory behind this is by keeping the pressure on, forces the air lock out through the reservoir. I have tried it a few times myself and it does seem to work
Craig Williams

This thread was discussed between 19/08/2008 and 06/09/2008

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