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MG MGB Technical - MGB brakes

Hi Everyone,

I rebuilt the master cylinder in my '80 LE and replaced the rear brakes. I also replaced all of the rubber lines in the system. Now, I can't get any fluid to come out of the rear wheel cylinders when I try to bleed the brakes. I was very careful when I put the master cylinder back together. My thoughts are that it's something with the proportioning valve. Any ideas are greatly appreciated.

Cheers,
Dave

David Plantz

Can you get any fluid out of the calipers on the front brakes?
Matt Kulka

Have you checked the steel brake line going along the axle? Often times this is crushed or pinched preventing fluid getting to the wheel cylinders.
Kimberly

David try screwing out the brake warning switch while bleeding the brakes it senses brake failure and closes off the weak circuit. Denis
DENIS

David, If you have a Haynes MGB manual check page 149 for brake bleeding on cars with a Pressure Differential Actuator. Their sequence agrees with what Denis has suggested. Clifton
Clifton Gordon

You could have clogged/corroded bleeder screws.
Glenn


Hi Everyone,

Thanks for the quick responses. I am able to get fluid out of the front calipers. A very small amount comes out of the rear wheel cylinders ( I replaced these with new OE cylinders). I also replaced all of the rubber lines. Now that you mention it, I broke the pressure switch when I was taking the master cylinder apart. I found a bolt that would thread into the pressure switch's hole instead. Could this be the problem? I know the rear lines aren't blocked b/c nothing comes out at the opening for the rear wheels on the master cylinder.

Cheers,
Dave
David Plantz

The switch is only supposed to activate when there is no pressure in one of the loops. Have you tried more "gentle" pedal? The bolt would only keep the shuttle from moving. It should NOT stop flow.

It is possible that you got the restrictor valve for the rear line in backward?

Hmmm. I'm not confusing the issue am I?

mike!
mike!


Hi Mike,

I'm not exactly sure that I know what the restrictor valve is. It has to be something that I put together wrong in the master cylinder. I just didn't want to part with the $220 for a new MC.

Cheers,
Dave
David Plantz

I've been down this path myself. did all the recommended things,bought bleeding kits etc pumped until. I thought my leg would fall off. Actually ended up getting the master sleeved, unnecessarally it turned out. Apparently MG used to bleed the rear brakes with a powerfull vacuume gadget. I now use a large syringe and a bit of tubing. I undo the rear brake line at the master, pump in about 50ccs of brake fluid and then do up the line again, THEN do the pumping out the air routine. It you don't have acess to a syringe you might try rigging up a drop out bottle on your vacuum cleaner. This is a largish air tight bottle with a long and a short tube stuck in the lid. The long tube (which goes to near the bottom of the bottle) attaches to the bleed nipple and sucks brake fluid. The short tube goes to the vacuum.
peter

Irrespective of how effective or otherwise it is at bleeding if you open the rear bleed screws, push the pedal and it gets hard before it reaches the floor and no fluid comes out then something is blocked! Since each chamber feeds one front caliper then if you can get fluid (and no back-pressure at the pedal) out of each caliper bleed screw then the chambers themselves must be OK. However the rear brake line uses a different tapping on the body to the left front caliper, with which it shares a chamber, so possibly that port is blocked. It is also quite close to the brake balance tapping, how long was the blanking bolt you used? Could it be blocking the rear port? It's a bit messy but what happens if you slacken the nut securing the rear pipe to the M/C body and push the pedal? That will prove it to the MC body or the rear pipework. Rear restricter valve? Whilst some BL cars of the era did have a restrictor AFAIK the MGB controls rear braking effort by the relative dimensions of the rear slave pistons, which is why GTs have larger pistons than roadsters, as they can stand more braking effort at the rear without locking up.
Paul Hunt

Not so long ago I repleced several brake pipes on another make of car. You've guessed it, no fluid at the rear. After making several checks found it to be ok at the ABS outlet but nothing at the rear caliper. This pipe had an in line joiner but unfortunately and unknown to me at the time when I made up one of the pipes the centre pin of the flaring tool broke off and remained in the pipe with obvoius results. Make sure that you have fluid at each end of the pipe.
Iain MacKintosh

I am talking about a return resrtiction valve NOT a proportioning valve.

A restrictor only works on fluid returning to the MC and is only used with drums. The reason and need is to keep the shoes close to the drum so you don't have a long pedal and the braking is more effective. It is not used for disc brakes because there is no need, the disc runout and piston seal "memory" creates enough space for the pads to float (and cool) but does not retract them all the way. Drums have all those springs and will pull the shoes as far back as physically possible. This can be a fir distance between adjustments!

I was questioning myself as I didn't remember but I KNOW many cars have them.

Mike!
mike!

David,

Lots of info. A bit hard to weed through it all, isn't it?

Since you have made a modification - the bolt in the pressure switch hole - that's the first thing I'd suspect. As Mike says, the shuttle sliding to one side shouldn't stop the flow, but then again, a bolt holding it in place may not factor into Mike's equation. Take the bolt back out and cut it off so it doesn't protrude beyond the threads on the inside. (Count the threads in the hole, and trim your bolt to match.)

Then, to get the shuttle to slide back to the middle, bleed a little fluid out of the front brakes. Now try the backs again.

No go? Sometimes the flare nut gets rusted to the steel line so that when you turn it, you twist the steel line. When you replaced the rubber hose or connected the master cylinder, did you inadvertantly twist the steel line? That could have made a crimp in it.

If that doesn't pan out, then I would progress up the circuit sequentially to find the blockage:

Loosen the fitting at the rear slave cylinder. Have an assistant pump the pedal. Fluid run out readily? If so, the blockage is in the slave cylinder or bleeder. If fluid still didn't flow, tighten that fitting and go to the next one up the stream. That would be where you changed the rubber hose. First loosen the end nearest the back wheels. Fluid flow out? If so, your problem was in the steel line between that hose and the slave cylinder. If not, tighten that fitting and loosen the next one up the stream. Continue this way until you find the place where fluid flows readily. That should point you to the area which is blocking the fluid.

Best wishes, and please come back with any more info, or to let us know where you found the problem.

Matt K.
Matt Kulka

This thread was discussed between 01/08/2004 and 04/08/2004

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