78 Big 10

[76shuvlinoff]

Hello, been a lurker here for a while. Yesterday a project followed me home.

I found a 78 Big 10, 6 cyl, manual trans on the floor, looks like it's all original with 24,008 miles. Needs some cosmetic love but for the most part it's a solid cream puff. I test drove it without issue but when unloading it at home the brake pedal went to the floor (power brakes). The brake light on the dash is on, I found the rear reservoir empty. Filling it did not solve the problem. I need to get it into the barn for a real look but I don't see fluid being sprayed around underneath or at the wheels yet.

With the engine running the pedal will hesitate but go to the floor every time. With the engine off it feels like it builds pressure and holds.

I've worked on a lot of new and old bikes over my life, I can spin a wrench but this is my first Square.

Mark

[95 S_Trucker]

There is a leak somewhere. A brake line rusted out, a wheel cylinder or caliper, or the rear seal on the master cylinder. If it’s the master cylinder, you should also replace the brake booster. Brake fluid will destroy the diaphragm inside.

[Bandit130]

Welcome.

I might think all brake system has not been replaced in many years.

This is what I would do

Replace master cylinder, front flex hoses, rear wheel cylinders. May not need to replace booster, but you can to be certain.

The front calipers may or may not need replacing, but can replace to insure front system is good. I would flush and blow out with air all lines to remove old fluid.

This is what I did to my 74 that had been stored for years. My light would come on too. After replacing everything, brakes are great and light has not come on again.

[76shuvlinoff]

Thanks for the replies. I crawled under and don't see any signs of fluid leaking. I pulled the mc , it's dry between it and the booster. I can actuate the master cyl by hand pretty easily (he says not knowing how difficult "normal" is) It holds with the engine off but bottoms out when I have the benefit of the booster. Right now I am assuming the mc is shot.

Yes I agree, I do need to flush the whole system

Thanks for the feedback.

[95 S_Trucker]

I would only replace the booster if the rear seal in the master cylinder was leaking. But it sounds like it’s not an issue on your truck.

[78 Big10 454]

I've had this same thing happen twice in my Big 10. Both times it was the master cylinder.

[76shuvlinoff]

I'm back, bench bled and replaced the mc, brakes seem good but I still have a "Brake" light on the dash. Pulled the wire of the proportional valve and the light goes out. I guess the next step is to bleed the entire system.

[smokehouse]

The good rule of thumb is if you don't know how old the brake system is replace it. This is safety we are talking about because if it is the og stuff the truck is over 40 years old it's time to replace. i just bought a 78 burb and i've been driving it and everything looks good even the rubber lines look good but it's all getting replaces very soon. In my experience you can't just replace some things and not the others because now that part that is 40 years old is now the weakest link in the brakes and they will blow sooner or later. I don't like rebuilding my own calipers either because now that's on me if i just buy rebuilt ones at the store that's on them.

[Bloodhound]

Originally Posted by 76shuvlinoff

I'm back, bench bled and replaced the mc, brakes seem good but I still have a "Brake" light on the dash. Pulled the wire of the proportional valve and the light goes out. I guess the next step is to bleed the entire system.

You need to reset the proportioning valve

[76shuvlinoff]

Originally Posted by Bloodhound

You need to reset the proportioning valve

I read up on that. I figured I'd bleed everything first if I can so I can be sure I got the bubbles out then move to that.

thanks

[Greasey Harley]

Originally Posted by Bandit130

Welcome.

I might think all brake system has not been replaced in many years.

This is what I would do

Replace master cylinder, front flex hoses, rear wheel cylinders. May not need to replace booster, but you can to be certain.

The front calipers may or may not need replacing, but can replace to insure front system is good. I would flush and blow out with air all lines to remove old fluid.

This is what I did to my 74 that had been stored for years. My light would come on too. After replacing everything, brakes are great and light has not come on again.

This.
Also the rear flex hose.

We got a saying around here "If the brakes don't stop you, something will"