Brake pedal goes to floor

[68gmcdude]

Hi All... Suddenly my brake pedal is going to the floor. My '68 GMC has manual drum brakes all the way around. The rear reservoir was low on fluid. Passenger rear looks damp, maybe wheel cylinder is leaking? Could master cylinder be bad? Just air in the line? What steps should I go through to find out exactly what's wrong? Thanks!

[custom10nut]

If you pedal goes down but the brakes still work, and the fluid level stays even, then that means your MC is Bad.
What you described is a bad cylinder on that rear wheel.
Since your front half of the MC stays full, that says nothing is leaking, up front.
When you pull that suspected drum apart you’re gonna find the diaphragms are bad and leaking.
If the cylinders are originals then it’s time to just replace them all (it is 53 years old, after all), and while your at it, possibly put a new set of shoes on.
It sounds like a lot of work (it’s really pretty easy, but then you’re not going the be revisiting the same problem on another wheel in the near future

[68gmcdude]

Thanks for the reply custom10nut. My plan is to replace all of the wheel cylinders. If one went the others are probably not far behind. Thank you for clarifying the MC issue. I wasn't sure about that. All of the pads don't look bad, but I might as well replace them and be done with it. I'll keep you posted.

[custom10nut]

Take a good look at all the components when you take them apart, and if you’ve had the truck for a while and have never replaced them, it’s time
One word of advice I have for you.
If yo start in the rear, take both drums off, but disassemble and reassemble one at a time.
This allows you to have a reference to look at to be sure you have it assbled correctly.
Then when you’re don’t with all four corners, get some help to bleed the air out (it shouldn’t be much) starting with the drum farthest from the MC.

[weq92f]

.

If you can pump on the pedal and get a higher resistance point ( firmer pedal ), you may also be fighting a MC problem.

I drove like this on an all drum '68 for a long time. Every time I needed a bit more pedal I'd just pump the brakes a bit to get it! Dangerous? Maybe.

-klb

[68gmcdude]

So looking at brake shoes online, looks like there are three different sizes for my truck... 11" x 2.06, 2.07 or 2.82. I'm guessing this is the width of the shoe pad? I measured my current pads at basically 2.06 with my micrometer. I'm assuming this is what I should order? Picture attached.

[Happy_dan]

Just an FYI - The bleeding sequence on my C-20 was not the furthest because of the way the brake lines run. Search this site for the right sequence because I can't remember and don't expect to need to do it again for a while.

I believe it's usually passenger rear because it is the furthers, but on my truck it was drivers rear first, then passenger rear then passenger front and finally drivers front. But I could be mistaken.

[MySons68C20]

Take a pic with your phone before removing springs etc......big help upon reassembly. I prefer back bleeding....bought a cheapy NAPA kit and it works great.

[68gmcdude]

I have another question. My front drums have the hubs attached to the drums. Looks like there are no replacements with this setup. I only see new drums without the hubs. Do I buy just the drums, have someone remove the hubs from the old drums, then add them to the new drums? I appreciate everyone's help!

[Wrenchbender Ret]

Originally Posted by 68gmcdude

I have another question. My front drums have the hubs attached to the drums. Looks like there are no replacements with this setup. I only see new drums without the hubs. Do I buy just the drums, have someone remove the hubs from the old drums, then add them to the new drums? I appreciate everyone's help!

There are 3 rivets holding the drum on. Cut the heads off the rivets & remove the drum from the hub. You do not have to replace the rivets. The lug nuts hold the drum on.
George