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MG MGB Technical - Bleeding Brakes!

OK, Tell me what I'm doing wrong.
I'm trying to flush out the brake lines (early Mk1) with alcohol prior to putting in fresh fluid. (car has been off the road being restored for 10 years so the brakes haven't been used)

Front brakes - one at a time, the old fluid came out and the alcohol flushed through by gravity quite nicely one the siphoning started from the fluid container at the master cylinder.

Now the problem - I slackened of a rear bleed screw - nothing coming out - pumped the brake pedal a few times - felt quite hard - still no fluid (even when I took the whole bleed screw out)

Is it a problem with the slave cylinder or something to do with the handbrake? I know I should have started with the rear lines, but I was going back to redo the front anyway.

Any suggestions??
John Minchin

Try the other cylinder, this will show if it is in the common pipe or the cylinder you have tried. If it is in the pipe I would vote for the flexible connector. If it is the cylinder you could slack off the nut on the input to the cylinder and if it nolonger has a pedal then its probably a blocked blled orifice or mmaybe the seals have dissovled and the pistons are jammed.
Stan Best

Have you replaced the flex lines? Check the steel line over the axle, they're known to get crushed be tow truck drivers.

Ah... this could be your problem- your 4 way valve for the brakes. there is a piston in it which seperates the front and rear brakes. i guess not really a piston, but it move back and forth if the brakes are not equalized, and triggers the brake failure light.

when you bled the front, the piston moved back, blocking the rear brakes. now fluid cannot flow through it for proper bleeding. it doesnt block it 100%, but most of it. it doesnt seem to block as much of the front in the reverse situation.

what to do? i fashioned a tool by cutting down a bolt that fits into the 4way in place of the brake switch. then i use a thin nail to push the piston in position, then screw in the bolt to hold it in place until after the bleeding.

you have to cut down the first 1/4 inch of the bolt, to the diameter of the size of the switch insert. sounds complicated, but it's not. it's fathers day, i've had a few beers, and probably not making it as clear as it should be.

don g

OK so I should do the following (in order) -
Check lines are not crushed.
Check other side rear and see if fluid passes through
Undo rear of flexible line - see if fluid coming through
If not release front of flexible hose and see if fluid coming through
If not check 4 way splitter (does it have a piston on early Mk 1 cars?)
If fluid coming through flexible line, check rear cylinders for damage / seizing

Anything else?

Thanks everyone for your advice
John Minchin

Why flush alcohol thru the brake system? I have never even heard of this til your post.
Bob Ekstrand

There is no piston in the 4 way splitter in MK I MGBs. Brake effort proportioning is done by system design. I imagine the one leading one trailing arrangement means it's unlikey you could ever lock up the rear wheels. I don't see why the brakes should not get a drink when it's John's round. The MK I MGB brakes are model of minimilist engineering doing everything you want without an ounce of fat on them.
Stan Best

The recommendation to flush lines with "denatured" alcohol ("metho" downunder) is in the BBS archives may times - to avoid mixing different and potentially incompatable types of brake fluid. Especially if you don't know what the PO has used.

Stan, once I sort the current problem I'm sure it'll be OK for a long time - not much can go wrong ;-)
John Minchin

FWIW brake proportioning is done by caliper/slave ratio on *all* MGBs, not just Mk1. In fact GTs have slightly larger bore slaves than roadsters as they can take more braking effort without locking with the extra weight (not V8s though, due to their harder suspension and possibly slightly different weight distribution they also have roadster cylinders despite being GTs).

If you have fluid coming out of the rear flex hose there is no need to check the splitter as fluid must be passing through that to reach the flex hose. Siezed slaves won't be a cause of no fluid, unless they are so badly corroded that it has blocked the ports as well. On old cars that have never been bled it can simply be hardened dirt in the bleeders.
Paul Hunt


The flexible line was OK - replaced with new anyway. The lines were all OK and running freely - no crushing.

So I removed the brake cylinder - gunk in the main bore and the bleeder passage was completely blocked - I had to drill it out with a twist drill. A light cleanout of the cylinder with steel wool has removed all the foreign matter and everything's ready to go - almost. I the procees I discovered the circlip that holds the cylinder in place is cactus and could have fallen off at any time.
Thanks for all the advice
John Minchin

As suspected probably a dissolved seal. Pleased to see you changed the flexible pipe while you were working on the system and thanks for the feedback, not everyone does that.
Stan Best

This thread was discussed between 15/06/2008 and 23/06/2008

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