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MG MGA - Replacing rear brake pipe

What should have been a relatively simple job this morning to replace one of my rear wheel brake cylinders has turned into a much bigger one, as the threaded union at the end of the brake pipe is stuck fast in the old brake cylinder and won't budge. I have tried every spanner known to man and now have a nut which is rounder than it is hexagonal, so now I need to replace the brake pipe as well.
In an attempt to limit the amount of brake fluid I lose when I disconnect the brake pipe from the 3-way adaptor I'm thinking of using a spare bleed nipple to plug the hole. Does anybody happen to know if the size and thread of a brake pipe union is the same as a bleed nipple?
Thanks in advance

Robert Sinclair

Robert. Over the years, I have seen several different sizes of bleed nipples used on various wheel cylinders and calipers. Hence, there is no guarantee that simply having one size bleed nipple would allow you to use it as a plug to replace the rubber flex line.

But, we know that the fittings on the rubber flex lines have to be made to the standard size, unlike the bleed nipples on the wheel cylinders. Hence, I would suggest that you obtain a new rubber line (how old is your current one?), and obtain a bleed nipple, bolt, or simply a rubber stopper, of the size needed to plug the line. Since the line is not going to be under pressure (i.e. you will not be stepping on the brake pedal), a simple rubber stopper should work fine.

Les
Les Bengtson

It depends on the thread used in the three-way. some early cars used a coarse thread, later ones are 3/8"UNF, which is a standard brake nipple size.

It's really worth getting a brake line spanner for jobs like this. It is almost impossible to round unions when using one.

Dominic Clancy

Not sure if it actually helps or just makes me feel better, but in similar circumstances I take off the MC cap, and place a plastic bag (or similar) over the open top of the MC before replacing the cap - so in effect taking away the breather in the cap and so hoping to create a vacuum to hold back the fluid.
Graham V

Thanks for the advice. I'm halfway through the job now, and so far so good. Put some plastic under the MC cap, as Graham suggested and used a rubber bung in the 3-way adaptor instead of an old bleed valve... and both worked a treat, with no perceptible loss of fluid. The main problem was sourcing the replacement pipe, as some suppliers only sell full sets and most others, including Moss, were out of stock with no idea when they'd be getting new ones in. Eventually I found one (the last one) at Brown and Gammons which arrived yesterday.
Thanks for the tip about getting a brake pipe spanner, Dominic. I didn't know they existed, but I've got one on order now which will definitely make life easier in the future.
Robert Sinclair

This thread was discussed between 13/07/2022 and 15/07/2022

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