Brake pedal goes to the floor. Help!!!

[klwebb]

Hey y'all. A few days ago I noticed my brake pedal moving a lot more than usual towards the floor, and every once and a while it would go all the way and I still wouldn't be stopped. If I pumped it a few times it would help but still not how it should be. I put some more brake fluid in and I thought that fixed the problem but it started acting up again today. I'm guessing i need to bleed the lines? But I have no clue how to do that. I'm not going to be driving it until it's 100% fixed, I could barely stop in time when I was parking in my driveway. It's got power brakes and power steering w a 350/350 combo if that helps at all. I'm still pretty new to the truck game and I'm learning as I go. Thanks in advance!!

[rich weyand]

If you don't see a loss of brake fluid in the reservoirs or brake fluid leaking under the vehicle, the master cylinder is probably shot.

There is a little rubber cup on the cylinder piston (and other rubber widgets) that age hardens, and then is not supple enough. The brake fluid you are trying to compress in the lines leaks past these rubber seals back into the reservoir. You don't lose fluid, but you don't have brakes.

This is common and happens to all of them past a certain age, and our trucks are all past that age. So if it has never been replaced, that is probably the issue.

There are three kinds: one with two equal-sized reservoirs, one with a larger reservoir in the front, and one with a larger reservoir in the back. You need one of the latter two, which one doesn't matter because the two fittings on the brake lines are different. One will only go on the larger reservoir, whichever one it is, and one will only go on the smaller reservoir. If you get one with the larger reservoir in the same place as the one you have now, you won't have to bend the lines to get them to the proper reservoir.

BTW, try not to spill the fluid when you change the master cylinder -- it makes a great paint remover. You can get most of the fluid out of the current master cylinder with a cheap turkey baster. Do not re-use it.

Once the master cylinder has been replaced and the reservoirs filled, you need to bleed the brakes.

[rich weyand]

This is what you got. The two black things are the first and second circuit seals, or cups, and they're probably shot.

You can also rebuild the thing and put new seals in it, but they're not that expensive, and, as you have found out, it is a critical safety component. Not for amateurs, IMO.

[rich weyand]

Oh, I just looked at the year of your truck. If it is a 1971 with drum brakes front and rear, the reservoirs will be the same size. Get whatever you have now.

The part is about $50. Labor is a couple hours if you take it to the garage. Maybe only one hour. It's a pretty easy job.

[klwebb]

Originally Posted by rich weyand

Oh, I just looked at the year of your truck. If it is a 1971 with drum brakes front and rear, the reservoirs will be the same size. Get whatever you have now.

The part is about $50. Labor is a couple hours if you take it to the garage. Maybe only one hour. It's a pretty easy job.

It's actually on my 1977 Silverado, forgot to mention that and forgot that it only talked about my 71 in my signature. Is the process still the same? Seems pretty simple except for bleeding the brakes. I'll probably take it to a pro anyway though.

[rich weyand]

On the 77, you do need the two different-sized reservoirs.

Changing the master cylinder is pretty easy. Put PB Blaster on the line fittings and the two mounting nuts. Take the lines off first, then the nuts that hold it to the vacuum canister. Reverse to replace.

https://www.youtube.com/watch?v=ZwdHUVhoEiA

If you bench-bleed the new master cylinder before you put it on, you probably don't have to bleed the lines.

https://www.youtube.com/watch?v=kEPXgDGU6_0

[friscobob]

You essentially have two brake systems, front and back, it would be rare indeed to have them both fail at the instant. In my book, brakes are number 1 before any other system, they need to work right or don't drive it till they do.
Bob

[rich weyand]

Usually what happens is one goes marginal and you don't notice in casual driving. When the other one goes, then you notice. The brake light should go on when one goes marginal, but that assumes that is still working after forty years as well.

Do not drive it without it being fixed, even to the garage. My truck has been very reliable, always able to go. But the one time it was untrustworthy in the brakes department, for a shot master cylinder, was the one time I had it towed. On a flatbed, since with a 4WD K10, I wasn't sure about towing it on the hook.