air in brake lines
 

After bleeding my brakes (67, drum, manual), the pedal pressure diminishes again over several weeks. There is no sign of slave cylinder leakage at wheels, nor at master cylinder, but somehow air is getting into the lines. Would it be appropriate to rebuild or replace the master cylinder?

If your bleading them correctly? My 65 coupe has 4 wheel drum manual & I had to bleed them several times. when I opened the bleeder screw I heard air hiss out.
If there blead correctly & no fluid leaks.
the master cyl would be the next thing Id check. The piston wears and fluid seeps past & pedal deminishes.
Hope this helps.

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Thanks for the info. I think I'm going to send my master cylinder off to be rebuilt, it seems like the logical thing to do. Thanks for your help!!

You may be better off buying a new (not rebuilt) Master cyl. Wheel cyl & master cyl.
hever seem to work after there rebuilt.
At least for me I always end up having to buy a new one! Ask around first-Don't go totaly on my openion!
Take Care!

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