Need to pump brakes on a hill

[englishmauser]

With my wife's M715, I need to pump the brakes when trying to stay stopped on a hill. The pedal will gradually go down. Otherwise they seem to stop vehicle O.K. and don't need to be pumped. The fluid level stays up and doesn't leak. Any ideas on what I should look at first to fix this. Thanks

[97XJSP]

Well, I'm no expert but I'm inclined to think it's the master cylinder. The other common cause of this is a leak, but if the fluid stays up then that obviously isn't it. There isn't anything beyond the M/C other than a leak that would cause the pedal to drop... Is it the original M/C?

[Flingarrows]

Yea, it sounds like the MC

[Dan Mac]

If the fluid isn't going down it is definitely the master cylinder. Anything else causing it to do that will lose fluid.

[syzygy]

I second (or third, fourth, whatever) that it's probably the master cylinder. I've been told that what's happening is that the internal seal on the pushrod is leaking. When you push down on the pedal the pushrod moves forward and pressurizes the lines. With a leaking seal that pressure leaks slowly back up into the reservoir, thus letting the pedal sink down and down and down until you run out of travel. I've also been told you can replace just that seal if you want, but I've never tried... the one time it happened to me I just got a replacement unit.

[N2S3]

Maybe another reason is water, brake fluid is hygroscopic. If the water gets hot near the drum then the vapor is compressible.
All the fluids should be exchanged, the lines too...

[Duane]

yup its the master. change it it will be fine.

[englishmauser]

Thanks for the advice, I'll change the master cylinder. I looked up on NAPA's website for a 1966 Gladiator pickup. I came up with part number NMC 36006. anybody know if this will work. Comments on the description says w/ dual rear wheels and 4 speed manual transmission.

[bob40]

did you try bleeding the mc before buying one

[Barrman]

Bleeding the brakes removes air. Air in the system causes a spongy pedal, however. The pedal will remain firm as long as the air is compressed such as after the pedal has been pumped 2 or 3 times.

A pedal that slowly falls is not caused by air. It is either an internal or external leak. Only the master cylinder can have an internal leak and fluid not lost from the system. Since it is the only part of the stock brakes that has 2 seals.

When you do swap out the masters, it might be worth your time and effort to change over to DOT5/BFS. My stock master went bad, I put on a brand new dual circuit Corvette master and 4 years later, it killed itself by rust ingestion. That was with me driving it 3-5 times a week. I put on another new one and swapped over to BFS. Still a shiny clean master 2 years later and brakes that work everytime.