MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Brake Pressure Failure Switch

I have ordered the rebuild set to repair a leaking brake pressure failure switch on my '72B. Assuming the brake lines come off easy, I intend to remove the brass block and work on a bench. Are there any special details of rebuilding this I should know about? The Bentley & Clymer manuals show a diagram with shuttle valve pistons and rubber seals that don't look at all like o-rings, in this block, was that the configuration of earlier models? I would like to have a good idea of what I am getting into, because the replacement block in most of the parts catalogues carry a heavy price of $300-$400.
Joe

The price of this thing is out of this world, no?

Somebody had earlier mentioned replacing the whole thing with straight fittings, but a source was
never given.

The illustration in the Bentley book is of the early style with cup seals.

Later models use "O" rings. There are 2 types of "O" rings used (small, and smaller).

Both types of "O" rings usually are included in one kit. The ones you eventually use depends on
the shape of the shuttle piston used in your particular unit.

Remove the unit from the car. Unscrew the switch from the brass body. Unscrew the hex fitting and
rap the brass body flat on a bench until the piston protrudes enough to gently grab it with a pair of
needle-nosed pliers. Pry off the old "O" rings, being careful to not scratch the piston. Clean all the
parts thoroughly with brake fluid and a Q-tip. Install the new "O" rings and dip the piston in brake fluid
before installing. Install the piston and center the piston detent with the switch plunger. Reinstall the
hex fitting, the switch fitting and switch body.
Daniel Wong

I think the following items could be used to replace the brake pressure failure switch.
Rear pipe connector Moss #182-500 and Three way connector #180-090. The rear
connector would connect the two pipes for the rear brakes. The three way connector
would connect the pipe from the master cylinder and the single front pipe to each
caliper.

I have not tried this bypass but it should work. I think all the fittings use the same
size thread.

Daniel; Where did you buy the kit? I talked to Gordon Strickland at The B Hive Saturday,
he doesn't stock the kits but he said he had ordered them in the past from Moss England
and they wouldn't fit. Moss US lists two repair kits but the but the second listing has the
following information. #181-995 Repair Kit, original assembly & assembly, listed above.
Based on the serial number applications it would appear this is the correct kit from 1970
to 74 1/2. Can anyone verify this to be a correct assumption. The Moss identification is
a little confusing, ("original assembly & assembly, listed above")

Regards, Clifton


Clifton Gordon

I checked my spare brake pressure failure switch and the threads for the front
brakes are bigger than those for the rear, consequently my suggested bypass
to use the three way pipe connector will not work for the front. Clifton
Clifton Gordon

Regardless of expense, my own personal opinion is that the best routes to safety in this situation are to either 1) replace the shuttle switch with a new one ($$$) or 2) find a decent used unit off of a later B and use it/rebuild it as per Danny's instructions above.

A new one will be more current in the parts books and the likelihood of being able to get replacement parts down the road probably better. The same is probably going to be true for the choice of a late model used shuttle switch such as Danny mentions.

I tried in vain to find a source of the cup seals that my 1968 unit would use, and ended up modifying my shuttle switch housing to become simply a pipe manifold for a single line system. This was supposed to have been a temporary setup, but it persists (inertia, and it works!). Anyway, there are some decent alternatives out there, and these are the ones I am aware of. FWIW.
Bob Muenchausen

Clifton -

For my '71 B/GT, I used Moss part # 181-995 (...now I know why I fold and file
away some of the old boxes and instructions that various MGB parts come in).

The kit includes 2 sets of pairs of tiny "O" rings, and a copper sealing washer
for the hex fitting, and illustrated instructions.

The piston with flat machined ends uses the larger "O" rings in the kit.

The piston with a dimple machined on one end, uses the smaller "O" rings.


The 3-way hydraulic connector for the front brakes is the hardest thing to find as it
requires a special shape to match the conic tubing flare used on the "B". One time
I spent an entire day rummaging through various brake fittings parts bins without luck.
Most racers simply replace the entire system with US standard double-flare fittings
and tubing. I was hoping to find a more convenient alternative to doing this.
Daniel Wong

Daniel; Thanks for the information. The one I need to repair is from a 71. I'm replacing
the original from the 68 GT with it.
Clifton
Clifton Gordon

This thread was discussed between 23/04/2002 and 24/04/2002

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.