Help! Front brakes keep going out!

[russtay66]

Hello, all,

I'm having trouble with the front brakes on my 69 F100 2WD. It's got power brakes - drum all around - 390 engine with C6 - Explorer Special.

A few weeks back, I lost lots of braking power all of a sudden. Got the truck home and found the right side cylinder had spewed. It was a new cylinder, replaced maybe a month prior. The left was fine, albeit older, so I decided to replace both front cylinders.

Worked fine for a few days, but less than a week after installing the new cylinders on both front wheels, the same thing: one of the new cylinders spewed all the fluid out.

Why would it be doing this? The brake lines seem alright (not rusted badly or corroded - at least on the outside). The hoses are also new. Would the master cylinder be the problem?

Any help is greatly appreciated. TIA.

Russ in TX

[Chasbod]

Hmmm. Are the cylinders spewing from the cup ends? If so, it would seem that the cylinders seals and pistons are traveling too far beyond the normal range.

The only thing I can think of is if the drums have been turned to much and brake pads to thin allowing too much travel? If all the linkage and hardware is intact, you might want to check to see if the piston/pushrods that link the cylinders to the brake shoes are normal.

I'm just thinking about anything that might have allowed the cylinders to overextend themselves.

Could be down the wrong alley but just a thought.

[russtay66]

Thanks for the quick response.

Yes, they are spewing from the ends of the cylinders. It IS possible that the drums have been turned too thin. They were just turned a few weeks before they first spewed. The guy who had it before me (a friend who works for my wife) replaced the shoes, but didn't turn the drums. I took it and had the drums turned, but I don't think they checked the thickness. They didn't say whether they did or not.

Can anyone tell me what the nominal thickness of the drum should be before they're considered too thin? TIA.

Russ

[Chasbod]

The max diameter allowed should be stamped on the drum somewhere. But it just seems that something else is allowing the the cylinders to overextend...if that is what they are doing.

What did they look like each time you pulled the drums off? Was everything intact? Rubber cups and pushrods still in place on the cylinder? All other hardware connected?

I don't know enough about power brakes to know if you can over pressurize the system to blow out the fluid. Hopefully someone else will add something.

[russtay66]

Yes, everything was still intact and connected properly. There was just fluid from the cylinders on the backing plate and tire.

The PO had wondered if it might be the master cylinder applying too much pressure, but that seemed odd to me that you could push too hard!!?? Your idea about over-extending seems more logical to me.

Thanks again for the quick response.

Russ.

[Chasbod]

Hopefully some other folks will be on soon that can ponder this too. If you have the wheels and brakes dissasembled, you might just take apart one of the wheel cylinders and inspect it. Make sure the bore is clean and smooth. I don't know if they rebuild these things or cast them new. Should have the inner spring, seals (in right direction), inner metal pistons and outside cups.

That's about all I can offer.

[72hiboy4x4]

is it possible the master cylinder or booster has been replaced, and it is pushing too much fluid into the system, thereby over-extending the wheel cylinders??
I don't even know if this is possible, but maybe??

[FORDification]

AFAIK you can't have too much pressure. The pressure in the line is going to push the cylinder pistons (and then brake shoes) out until it meets resistance (the drum) and then it's going to stop. Pushing extra hard on the brake pedal isn't going to have any adverse effect.

I'd have to agree with the thin-drum diagnosis, especially since you say this started right after getting the drums turned down. Of course, this is assuming you got the correct cylinders when they were replaced.

[lobuck76]

I'm thinking it could also be the wrong cylinder links. If they were too short the cups might travel far enough to loose their seal.

[Heavenlyfire]

Maybe, just another guess . If your springs are wore out, and the drums are too far gone, it could happen. Alot of poeple ignore the hardware when they do brake jobs. If the springs don't have enough tension they won't hold everything together like it should be. Like I said, just a guess.

[fordman]

i would say thin drums also of course if the w cylinders are new they could have tried to use less metal and make the cylinders too short. some aftermarket stuff isnt quite up to specks that they should be. and it could be lose sprongs also but the drums would also have to be thin in order for the shoe ro travel to far. you could also look for a small fracture in the line under the nut or at the flare.

[dragtruck]

To answer the pressure question,yes you can have too much pressure,1500 psig is max on brake systems,smaller bore master cly your pedal gets hard and moves less but produces more pressure ,a larger bore cly has a softer pedal and more travel and has less pressure at the same pedal ratio.If you keep wasting front brakes.check the rear brakes too,if the rears don't work then the fronts will do more work.If the drums have been turned too far ,then you will get brake fade alot sooner and they will worp or crack.Always buy new brake parts,rebuilds leak while still in the box and make sure the wheel cylinders are the correct bore for your year/application.The rear brake lines on our trucks(the hard line) tend to get damaged,rust and clog up.Go to one of the major brake kit companys and check there tech section,they have some really good articles.

[russtay66]

Finally got the brakes fixed. The problem most definitely was the drums. They had been turned too much, then turned again. Got the truck back yesterday, and it stops great, so far. Put new drums, new shoes, and new cylinders, and all seems to be well. Even got the exploded radiator fixed while it was at the shop. That's why it took so long to get the brakes done: the radiator blew up a couple weeks after the brakes went soft. Runs much cooler and stops much better. Coolage!

Thanks for everyones suggestions and help. Forums are great!

Russ

BTW this F100 is now for sale

[flyboy2610]

Glad to hear you got everything fixed!

[russtay66]

Or so I thought! It went out again on the way home from work today. The front right cylinder again.

What else can it be? What does the little block on the frame do? It's just below the master cylinder.

Thanks.

Russ