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MG MGB Technical - Brakes, refil and bleeding

I'm having difficulty refilling and bleeding the brake system and I believe that there may be damage/wear to the master cylinder but haveing not done this before I can't be certain so I am looking for advice from the more experienced.
It is a Tandem master with pressure failure switch circa 1975 ex Californian.
The problem- When I depress the pedal (bleeding nipple open)and let it come back slowly (bleeding nipple closed), the master doesn't refil from the reserviour.
With the brake lines detached from the master the reserviour will drain freely (ie no blockage there) and the lines are clear and have no leaks(I blew air). The front lines are new.
When I depress the pedal there is sometimes an initial linepressure/resistance which feels like I'm pushing fluid through the line. This pressure will suddenly decrease with a 'gasping" sound.
Background-
-I drained and removed the master some months ago.
at the time it was not easy draining the system, requireing much pumping of the brake pedal.
The brakes were working perfectly up until that point.

Is there a sequence to bleeding the brakes ie the back ones first then the front? could this be the problem? I started with the front by chance.
It seems that the piston cups/seals are not forming a seal, it fits the symtoms (no suction no, pressure). maybe have dried out (but it seems unliklely to me though).
Is it easy to renew seals (ie no special tools required) or should I simply buy anew (keeping in mine I simply hate to spend money (Bah Humbug Merry Chrismas Mr. Marley)
If you believe that the master should be replaced, should I replace the servo with it?
Happy new Year
Peter
Peter

Peter,
My first consideration would be safety not cost. There can be places to make dollar savings but your life is not one of them.
If you are not totally confident in doing this yourself then take out the master cylinder and servo and take it to a brake specialist.

Having said that there is a sequence for bleeding which is doing the most distant from the M/C first and the closet last. In this case: Rear Pass side, Rear Drivers side, Front Pass side and finally the Front Drivers side. The aim is to purge air from the lines and if you bleed a closer wheel first there is a chance all the air may not be removed.

Faulty seals usually just let the pedal sink straight to the floor with little if any resistance.

Do you have a manual or can you get access to one? This may be well worth doing.

Cheers, Pete.
Peter Thomas

Thanks Pete,
I'll try the back first sequence and it that dosen't work tackel taking the master out again. I have just got the wretched pedal box back in the right place and it was not easy as whoever did the RHD conversion got it slightly out of alighment, given up on alighning the correct threaded holes and drilled holes at random. It also seems origionaly to have been designed by someone with fingers 10 inches long but thats MG's for you. Is is possible to remove the master and leave the pedal box in place?It would seem reasonable.
Peter

Peter,
Normally I would say YES but going on what you have just described I am not so certain!
It should be straightforward enough though.

I know what you mean with the 10" fingers!
I had a mini cooper many years ago and so help me I swear the thing was assembled by pygmies.
No way my huge mits would fit in there.

That was where I first learned of the "joys" of having a M/C seal fail approaching a set of traffic lights in Melbourne.
Pressed the brake pedal and straight to the floor!
No vehicle damage thankfully.
Good luck,
Cheers, Pete.
Peter Thomas

Peter
I have wasted many hours, no days on brake repairs and I have to agree with Pete brake repairs is not the place to save a few bucks,IMHO take it to brake repairer they know exactly what they are looking for and will fix it properly in a fraction of the time and you probably get a warranty as well.
GORDON

Pete, for what it's worth, I had the same problem when I rebuilt my brake system on my 78. I couldn't get the master cylinder to pump fluid. It acted as if the piston was not sucking fluid from the reservoir. I finally got it working by, "priming the pump" by filling the master with brake fluid while off the car and using my fingers to plug the outlet holes, pumping the piston with a wooden dowel until fluid was spurting out the outlets past my fingers. It takes a while sometimes to get everthing working so don't give up.
Jerry

Peter. "Bench bleeding", as Jerry describes, is a good idea. It is also a good idea to make sure you are getting full piston travel so the seals will clear the small holes going to the resevoir and allow the fluid to move downwards. It is fairly easy to remove the master cylinder from the brake servo unit, if you have the NA configuration system. Do not know if any of that was changed when the RHD conversion was done. If it is standard, simply disconnect the fluid lines and the two nuts holding the brake MC to the servo and move the MC forward. Rebuilding is fairly straight forward on this system. But, there is a balance rod on the left hand side of the MC which acutates the brake failure warning light switch. Most people fail to rebuild that sub-system and get air leaks through the switch, which is not sealed. New MCs are available and, as others have pointed out, your life is worth it. If the bore of the current MC is bad, either have it re-sleeved or get a new MC. Les
Les Bengtson

Thanks Guys,
I have decided to go buy a new master.
I took the old one to pieces and it was in very poor shape indeed, corroded and full of debris (and I thought they were good brakes!). I'm looking forward to simply tremendous brakes with the new master.
Jerry, good tip about priming the master, I'll bear this in mind when fitting the new one, we use large 50ml syringes where I work and these might be just the thing.
Les, I considered reconditioning the master (preferred option cost-wise) but unfortunately the reservoir is cracked (probably where all the debris came from) and I am told that these can't be bought independently of the master.
I got a shock on the price though, one guy wanted over $600 Australian. I eventually found that Triumph and Rover spares in Adelaide would sell me one for $375 which although a big improvement is still a little breathtaking.
Not as breathtaking as approaching an intersection with no brakes Pete!
Peter

This thread was discussed between 01/01/2003 and 02/01/2003

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