One question in 3 parts:
1. Is my master cylinder bad?
2. Should I replace it or rebuild it?
3. What advice do you have if I rebuild it?

'74 Swinger with drums all around, non-power brakes, all stock. When I'm stopped at a light, the brake pedal slowly creeps down toward the floor. Stopping seems normal except maybe with a slight pull to the left. I flushed and bled the system when I got the car (last fall, after it sat parked for several years), and at one point I did let the MC go empty...oops. Is it possible I just have air in the MC and need to bench-bleed it, or is there an internal leak?

If the MC is bad, should I rebuild or replace it? I'm not an experienced car guy and I've never replaced or rebuilt a master cylinder before, but I have a shop manual and it seems pretty simple to rebuild a MC. After reading about so many problems with remanufactured ones, I'm leaning toward the rebuild. Also, my MC seems pretty solid other than a little surface corrosion. Most of the car is very solid--it was garage kept forever, although not driven for the last several years until I got it.

Finally, if I do the rebuild, what is your advice other than to follow the manual?

rockauto.com has a new one on close-out for $14.95.

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Joshua

...and a rebuild kit for $11.95. All this stuff is pretty cheap, so it's not about the cost so much as what's the best fix.

Get the new one! Hurry!

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Joshua

Also note that any one (or more) of your wheel cylinders might be bad instead or as well. That will also cause the pedal creep/drop that you describe.

Lou

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Home of Slant6-powered fun machines since 1988

But if that's the cause, then wouldn't I also be losing fluid into the drums?

Yes.

Lou

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Home of Slant6-powered fun machines since 1988

I rebuilt the 83 D150 OE Master cylinder with a kit from Rock Auto,,

actually pretty EZ,,

Buy new stuff.

Car sat unused for a period of time, master cylinder most likely pitted and is letting fluid bypass its soft parts, and same for wheel cylinders. Replace everything, its your brakes and a lot easer to do than replacing all front sheet metal, radiator, etc. after a crash.

One can replace master & wheel cylinders, shoes as well as soft lines for less than a two hundred bucks resulting in a new breaking system.

Keep in mind drum brakes tend to get out of adjustment rather easily becoming unpredictable in emergency situations vigorously pulling in any direction. Having all the brake parts working correctly reduces hard braking unpredictability, and reduces stopping distances, which compared to today's cars is excessive even when in factory fresh condition.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

It turned out WJA Jr was right: there was a good amount of rust and pitting along the bottom of the cylinder bore and it needed to be replaced anyway.

Along the way, I found out the MC that was installed was probably not the right one anyway--for cars with drum brakes the bore should be 1+1/32" bore, but my old MC had a 15/16" bore. See this post for more info on that.