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MG MGB Technical - Rear brake bleed

I'm changing brake fluid on the '71 B roadster (dual line, no servo) and have run into a small snag. Since I'm simply trying to do a fluid changeout, I decided to start with the rears, whilst leaving the fronts for 'stage 2'. I'm using my usual method for brake bleeding, which consists of an assistant depressing the brake pedal while I open/close the bleed valve at the wheel cylinder. It's been a while since I've done this, so I may be a bit fuzzier than I should be. In any case, I'm getting little to no fluid out the bleed valve(s) at the rear wheel cylinders when the pedal is depressed. My theory is that since the fronts are still 'solid', the pedal can't travel very far, and consequently, the master cylinder isn't really being exercised enough to expel much fluid (my assistant reports a 'hard' pedal, with very limited travel). Does this sound right? If so, my approach would follow one of two generally accepted scenarios: 1) follow the 'gravity bleed' method, or 2) use a suction device at the brake cylinder bleed nipple to pull the fluid through. I guess a third solution might be to 'un-bleed' the fronts to allow suffient brake pedal travel to actuate the master cylinder sufficiently.

I'm thinking that the reason I've never had this problem before is that I normally am doing a complete bleed job (front and rears need bleeding), and since the fronts normally need bleeding whilst the rears are being bled, the pedal gets sufficient travel to push fluid out the rears. Anyone have comment on the above or insight into the solution? Am I on the right track? Comments and advice are much appreciated!

Dave Tye
'71B roadster
Florida, USA
David Tye

Not having the luxury of a 'good' assistant, I have reverted to using 'a suction device' as you call it and have found them most acceptable.

You might also try the 'gravity method' first of all though, because if you arent getting any fluid back there (flexible pipe collapse?) then you are wasting your time with buying yourself 'a suction device' in the first instance.

Good luck

Paul
Paul Barrow

Are the hard lines along the axel crushed? How old is the flex line to the rear? Are the rear brakes properly adjusted?
Kimberly

The fronts have nothing to do with it. The pressure should blow fluid out to atmosphere when you open those bleed screws. I would vote for blocked bleed screws, try removing one completly, does fluid leak out? Can you see through it?
Stan Best

OK, judging by the responses, I should've included more info on what's been done thus far. My bad.

The first thing I did was replace the flex line, and yes, it appeared clogged. Sadly, no help. I've also disconnected and/or removed all lines all the way to the master cylinder, and blown through all of them (including the bleed nipples themselves). No obstructions. I've tried to bleed the master cylinder while on the car, attach the first brake line to the master cylinder, and force fluid through it (holding my thumb over the opening on the brake pedal up-stroke). Nothing. Mind you, I do get somewhat of an initial spurt of fluid on the initial down-stroke of the pedal, but subsequent down-strokes produce very little...which coincides with the pedal getting 'harder' as brake pressure in the forward system builds.

In Stan's posting, he said fluid should flow out the rear bleeds when they are opened, regardless of whether the fronts are bled. Should the brake pedal sink to the floor, or near it, if the rears are opened and the fronts are already 'bled'? Mine does not. When the pedal is depressed, with rear bleeds opened (or the rear lines disconnected at the master cylinder), the fronts actuate and pedal goes about an inch or two...normal travel. I'm flummoxed.

Dave
David Tye

I was thinking about my single line system, your split system is a bit different. You will always have a pedal even with an open bleed screw at the back, thats the whole idea of it. Sorry to post without asking which revision of brake system your car has.
Stan Best

I thought the whole point of a split braking system was that the pedal *didn't* go to the floor if one circuit was open to atmosphere, otherwise the system seems somewhat pointless. I would expect the pedal to go down a bit further than normal, as the shuttle valve moves to indicate brake imbalance and the two parts of the master piston move relative to one another. This *should* push some fluid out of the open circuit, although probably not as much as on a single braking system. The manuals use the same section to describe bleeding for both single and dual circuit systems. You will have to centralise the shuttle after bleeding by rebleeding the first circuit very slowly until the warning light just goes out. If you go too far and it comes back on you will have to rebleed the 2nd circuit until the light just goes out, etc.
Paul Hunt 2

Thanks very much Paul. I suspected that something like what you described is the case. I assume the shuttle valve is located in the brass manifold looking device, which I can physically blow through (the rear line). I think what I'll do is bleed the rears using the gravity method, or use the suction method...I think the 'might vac' is available here locally. Hopefully the shuttle valve will remain centered and won't enter into the equation. Thanks again for your reply and assistance.

Dave
David Tye

This sounds like a good application of the easybleed valves. Just open all 4 at first? Maybe not, I guess the pressure would go out the least resistance and not the rest. Oh well. Made me think about it anyway.

Maybe let some air into the fronts to let the rear have some travel, then bleed the fronts last, as usual.
Tom

This thread was discussed between 18/07/2007 and 20/07/2007

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