I did a search in this forum before posting but didn't find exactly what I was looking for so I'm hoping someone has an idea.

2 yrs ago +/-: ABS unit starts activating every time I press brake at low speed. It makes the noise and I feel a constant back pressure on the pedal. I'm ok with it - brakes still work and I generally don't drive so I need the ABS to save me. ABS is still working, it just works all the time at low speed instead of only when it should work.

Today: Shift from park to drive and the brake pedal goes to the floor. No more ABS noise or back pressure. The truck still stops but not as well as it should to give me any confidence. There are no leaks anywhere, no puddles, and the reservoir is full, I haven't touched the brake fluid level since last fall when I replaced the fluid. The past few weeks I have smelled a faint wiff of brake fluid when I exit the truck. It seems related suddenly and the fluid in the reservoir is dark colored.

So I'm thinking some kind of failure in the HCU or master cylinder? The truck isn't going anywhere until it's fixed so I could start removing parts. Just would like some insight into where to begin. Thanks for your help in advance.

 

I would take a good hard look at all the rubber components. Brake hoses, calipers, ETC. also make sure you don't have a stuck caliper. Usually with an HCU/ECU failure you will get an ABS or Red Brake light on the dash. You could have a leaking master that is bleeding back into the booster. If you let off the pedal and press it again do you have a firmer pedal?

 

will do thanks

 

I added more info to my suggestions. Take a look at those

 

That worked! I reefed on the brake pedal and it seemed stiffer momentarily - but now the red brake light is on, I DO have a leak, and the fluid level in the reservoir is half. The last remaining flexi line into the HCU gave up. I'll make up a new one of those and see where it gets me. Thanks!

 

it probably had a pinhole leak and when you stomped on it, she gave way

 

yup

 

While you have it apart, you should really take the time and replace those front brake hoses if you haven't already. You've already got air in the system, so you may as well put another 40 bucks into it, IMHO.

 

Ageed, hoses last as long as a set of good semi metallic pads do before they begin to break down from the inside out imo. The F150's are the dividing line when it comes to pads. Semi Metallics are a must for F250/350's, but should be run on most 150's as well that haul anything. You can most likely get away with Ceramics or Hybrids on the V6's that don't haul much.

Anyway, that's a good way to gauge when you need to update lines. Once you go through a good set of Semi Metallics (like say NAPA pads), change the lines as well, at least on the front.

 

excellent advice - i replaced them last fall

and in the process found out what happens if you try to heat the end of a brake hose too much