'85 F250 IDI RCLB T19 4x4, aprx. 150,000 miles on truck. Former farm truck, owned it about five years, but have only put maybe 2000 miles on it. Been driving it all week since I'm in training across town. On the way home today, the brake pedal went soft. Still had pretty good brakes, but excessive pedal travel.

Found a parking spot, popped the lid (it's one of those old split metal reservoirs M/Cs), and the rear reservoir (rear reservoir is the smaller one up front on the M/C, right?) was empty! Refilled, pumped the brakes, got a little more pedal, but still soft. Looked EVERYWHERE for leaks, found none. INSANE downtown traffic didn't help, but prudence and downshifting got me home.

I didn't remove the drums, because it's an early '85 with the Dana(?) axle, where you have to pull the bearing and all to remove the drum. But pulled both wheels and with repeated brake pumping after refilling, found NO sign of a leak anywhere. Not even the slightest pinhole leak. Main line, split lines, soft line, prop valve, all dry. I figure even if it was a wheel cylinder, to lose that much fluid, I'd expect to see it roll down off the drum.

So I bled the brakes using the wife-assist method, got a LOT of air out. Fluid level is maintaining, pedal feels better, even better than before the pedal drop. I can lock up the rears in a panic stop, so I don't think the shoes/drums are sodden with fluid.

So why the fluid loss? I know the front level goes down over time because the caliper pistons inexorably advance to compensate for pad wear. But nothing like that should happen with the rears. Even if the rear mechanical adjusters fail (which I know is common), that should just result in reduced shoe/drum contact and weak rear braking, not in greater pedal travel or fluid drop.

Obviously I'll be very watchful of it in days to come, but what could cause the rear res to empty completely without an external leak? I admit I haven't watched the level closely, because we drive the truck so seldom. Lesson learned there, but the loss is still a mystery. Ideas?

 

it could be leaking into the power brake booster from the rear seal on the Master Cylinder itself, I had a Chevy C20 that did that to me

 

I'm guessing you have a slow leak that is evaporating or soaking into the shoes and not running out. And today it got low enough to suck air in and make the pedal go soft.

And, by the way, the only problem I've had with brake fluid on the shoes is the brakes have a tendency to grab.

 

Was the leak check done before the bleeding? Did you then repeat the leak check?

If leak checked only before the bleeding, any leaks may have been hard to find. The lines were full of air, with no fluid to leave evidence.

Also, the pressure inside the lines is going to be lower with compressible air present. You'd need incompressible liquid in the lines to reach maximum pressure. If a leak only occurs at higher pressure, it may have stayed sealed if some air was still in the line.

I'd suggest keeping a close eye on the level of both reservoirs. If not leaking out via the booster, as previously mentioned, fluid could be migrating from the rear system to the front. Because the front reservoir is bigger, it would be hard to notice a leak from the rear. Any increase in the front reservoir would indicate an internal leak at the master cylinder piston or at the proportioning valve.

 

Good points, thanks. Still not seeing a leak after bleeding, but as you say, I'll be watching now. Rear-to-front leak sounds like a possibility. I'm doubtful of a leak into the booster; I would expect the reservoir closer to the booster to be the one that goes low.

Thanks again all; will just remain vigilant.

 

It's going to be the rear wheel cylinders. Now that you've refilled it will become noticeable on the backing plates as the "inards" are full/saturated.