Got in the truck the other day to go to town, and when I get to the end of the driveway the brake pedal goes straight to the floor, no resistance at all. Good thing I'm used to downshifting coming to a stop. Anyway, get the thing back home and the master cylinder's empty, and no sign of a leak anywhere. I had last driven it three weeks prior with no problems.

Refilled master cylinder, worked the brakes, and refilled again, let sit. Checked two days later, refilled master, checked for evidence of leak and found nothing. As it sits right now, I've got about a quart of fluid into it, very spongy pedal, ( I know I have to bleed them) and still no evidence of a leak anywhere. I could bleed them, but where did the fluid go in the first place?

 

Mark

Check under the dash where the brake shaft goes through the firewall. It might have leaked into the cab, although that's alot of fluid. Check under the floor mats and see if the carpet is wet.

 

 

You don't have a woman who is mad at you, do you?

Rear drums you might have to pull the drums off and check the wheel cylinders. Then check the hoses back there for cracks or damage. Follow the tubing forward to the master cylinder checking for rust holes or cracks. Then on the front axles check the calipers for weeping piston seals. Again check hoses and follow the steel tubing up for rust holes or cracks. Hydro booster is the only thing inside. The master cylinder is on the front of the hydrobooster so it would leak in the engine bay. 

Checked drums and front calipers, no sign of leakage. No puddles on the floor, or evidence of a leak under the truck. For the amount of fluid I've put in this truck I would think that I'd find a puddle or some kind of sign of a leak. Not sure how I could go from no sign of a problem to a completely dry master cylinder without any sign of a leak.

Defiantly a head scratcher.

 

Mark

6 hours ago, Scarecrow said:

Anyway, get the thing back home and the master cylinder's empty

 

When the master cylinder is low, the fluid will be split into two separate reservoirs.  One should have been empty and the other should have remained at the top level of the dividing wall.  If you observed which one is empty, it would help you diagnose the problem.

 

Be sure to check the rear brake line between the frame rail and the fuel tank.  Since you live in the rust belt, your truck is a candidate for a brake line rusting through from the outside.

 

- John

If you haven't fully bled the brakes back up then you might not see a leak for a while as if at the point of the leak there is a air bubble then no fluid will leak out

 

I'd be bleeding the brakes fully then start the truck and either get someone to hold the brake on or wedge it down ( I use a 2ft breaker bar wedged inbetween the seat front and the pedal) and then check everywhere for a pressurized leak which will be easier to spot

Both of my trucks have had the brake line that goes around the front rust out, my 3500 happened coming up to a crossroads at some speed and I'm on the STOP road that was interesting to say the least, on the way to the trucks yearly check, our MOT, my 2500 was on the brake tester at the MOT check, both now done by myself in copper

Also done the one behind the fuel tank on my 3500

Annnnnnd the winner is..... Tractorman! Yup, brake line between tank and frame was rusted and had a hole in it. With all the dirt and gravel in there it acted like a sponge for the fluid, took it a while to drip. Now I just have to see if I can run a new line in there, or if I have to drop the tank to fix it.

Thanks for all the advice/suggestions, now I just need some time to get this done.

 

Mark

Dropping the fuel tank is rather easy as pie. You could sling the fuel tank in two ratchet straps between the frame to just ease the tank away from the frame to gain access as well. Heck I just done a fuel tank for a AirDog 150 install and the brand need fuel gauge sender installed. Even with over 10 gallons of fuel in the tank I was able to pull the tank up in place with the same two ratchet straps and bolt it up. Easy...  

 

Don't both with a floor jack or anything else being as the fuel moves its need to be stable on both ends and ratchet straps do really well.