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MG MGB Technical - Brake MC problem

Hi,

I posted a couple of days earlier with a problem bleeding the rear brakes after rebuilding the master cylinder on a '79 MGB. I have traced the problem back to the master cylinder. As it turns out, I can remove the brake line to the rear brakes from the master cylinder, pump the brakes, and still no fluid comes out at the MC. Is there anything I could have done wrong or missed during the rebuild? I'm pretty sure I didn't get any of the seal lips flipped the wrong way during installation. Could the differential pressure sensor cause this problem? Any other ideas before I take it back apart?

Thanks,
Rodney
Rodney Isom

Rodney,

Depending on how you first bleed the brakes the pressure differential switch could of locked out the rear brake. A working pressure differential switch for that year is suppose to do that if there is a loss of pressure in one of the lines.

Whenever your bleed the brakes, you need to unscrew the pressure differential switch, (use 9/16 open ended wrench) partially, 5-6 turns, so the brake switch will not lock up the shuttle cock plunger inside the MBC when you are bleeding the brakes. You use the open wrench since the plastic bottom of the switch wont let you slip the enclosed portion wrench onto the switch.

Aside from that, make sure the two-part reservoir is fully topped off. The front part of the reservoir (you can not see the interior of it from the filler plug) services the back brakes while the back portion (the one you can peer into) services front brakes.

I failed to do that during a rebuild/bled of the 79 B MBC. I spent the day, blowing into tubes, taking apart the entire rear brake system piping, hose etc to figure out where the blockage was.

The next day, I realized (when I peered into the reservoir) that the backend of reservoir was a little low in brake fluid, but the front end of the reservoir was totally empty. Topped off the reservoir then bled the rear brakes with no problem.

John Long
J Long

I was under the impression that the differential pressure sensing device only lit the light to warn you of pressure loss. Does it actually lock out the offending circuit of the braking system? If so, how do you reset it? By the way, I checked the reservoir & it's full.

Rodney
Rodney Isom

It doesn't 'lock out' the offending circuit, but once the shuttle has moved from its central position because of a defect causing pressure differential so activating the switch you have to fix the problem then recentralise the shuttle. This is done by applying pressure to the brake pedal with the ignition on, which should illuminate the warning light. Open a bleed valve on the circuit that *didn't* have the problem slightly to let some fluid out until the warning light goes out. If you let too much out the shuttle will move right across and illuminate the warning light again.
Paul Hunt

Paul

There are two different types (per the AP Lockheed MBC rebuild kit instructions) of the PDAS (pressure differential activation switch, i.e., shuttle cock) in the late B MBC.

You are correct as to the earlier version where the brake light switch (underneath the MBC) projects into a centralized depression on the shuttlecock. There is no "lock out" feature for this earlier version (I think 1976-1977)

The latter PDAS version used in the 78-80 B has a shuttle cock wherein the electrical brake switch protrudes on to a raised ring on the central portion of the shuttle cock. Once the shuttle cock moves in relation to a pressure difference in the front/back braking circuits, the ring moves as well to allow the metal pole of the electrical switch to protrude forward to hit the non-ring portion of the shuttlecock and to rest against the side of the ring. By resting against the side of the ring, the metal pole of the brake switch then holds the shuttle cock in its non-centralized position thus locking out that brake circuit which has less pressure (fluid) in it. When the metal pole or protruding tip of the switch is extended, the brake warning light also goes on.

Again, to relieve this situation, the electrical brake switch (i earlier refrered to its as a pressure differentail switch) needs to be unscrewed (by 4-5 turns) from underneath the MBC. This action removes the metal tip from the side of the shuttle cock allowing for the shuttle cock to recentralize again (via a shuttle cock spring).

John Long


J Long

Hi John,

I'm glad you wrote in....that made a lot more sense in relation to what I saw when I was rebuilding the thing. It looked like the shuttle had a spring return & I couldn't see how if the sensor got actuated, it would ever be able to return to normal without removing the switch. Actually, this wasn't my problem. I had the primary & secondary return springs reversed & the primary spring (in the secondary position) wasn't strong enough to return the secondary piston all the way. The piston was only going back far enought that the seal was blocking the secondary inlet from the reservoir so no brake fluid could get sucked into the MC. I swapped the springs (which look the same, I don't care what the manual says) & it works fine.

Thanks for the information,
Rodney
Rodney Isom

Rodney,

The two springs have different colors (white/black. Johnathon Twist has a ditty "black goes in back" (I think, thats how it goes) to keep the order of the spring insertation straight.

Glad you found the solution.

John Long
J Long

I also replaced my master cylinder with a rebuilt one that I purchased at my local auto parts store. I did unscrew the brake failure sensor prior to bleeding. But when I remove the rear brake line from the master cylinder, no fluid comes out while pumping the brakes. Any body have any ideas?
John Taylor

Take your rebuild back to the store, and get another one. As Rodney pointed out, the seals or springs could be screwed up by the rebuilder.


John
J Long

This thread was discussed between 21/05/2002 and 28/05/2002

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