Simple Brake Question

[bghnkinf350]

Too hard to think right now. I replaced the brake hoses on my truck and fluid was pouring out of them while I was doin the change over. Put it all together and bled the brakes. Now, while standing on the pedal, it is firm, but if I let up very slightly it will sink to the floor. Generally I would consider this to be a bad master, but this started after I did the lines. Could this be a sign there is still air in the system?

 

[Knuckledragger]

Probably. Do the brakes feel spongy after pumping them up? It also could be entirely coincidental that the MC went bad at this time. Try bleeding again.

 

[bghnkinf350]

Thank you. They are hard after pumping them up. I am nervous that it is the MC, but what do I have to lose by bleeding them again?

 

['94IDITurbo7.3]

this is while the engine is running right?

 

[bghnkinf350]

It happens with it running and with it off. When it is off, it is more pronounced since I depleted the hydro boost assist.

 

[itsacrazyasian]

theres a good chance the master is bad now if the system hasn't been bled in a very long time, especially if the fluid is very contaminated. Since the piston in the master doesn't normally travel past a certain point, once it travels its full length (when you're bleeding), dirt in the bore tends to damage the orings. which leads to a leaking/bypassing master.

edit: by all means try bleeding it again. fluid is cheap.

 

[bghnkinf350]

Thanks, I will bleed it this weekend. Not much air came out when I bled it.

On a side note, I had another question. I have a SD master on my hydroboost and SD brake pedal. How would I fare putting the F350 master back on if the SD master is bad?

 

[itsacrazyasian]

the f350 master will work but you will not obtain the same results with a smaller diameter bore master cylinder. its the basis of hydraulic theory.

 

[VStorm]

Most likely culprit, RABS valve.

If the master is bad and leaks internally, you probably will not get a firm pedal while standing on it. A very common problem that has confounded many people over the years occurs when one of these vehicles gets a fluid change or heavy bleed and dirt in the old fluid ends up on the dump valve seat in the Kelsey Hayes RABS module located under the drivers side on the frame rail. One way to try and salvage it is to adjust the rear brakes, bleed the system conventionally in this order: master, RABS valve(has a port), then each wheel in this order RR, LR, RF, LF. If your pedal still has the same stout feel when you stand on it(you are seating the dump valve with hydraulic pressure despite the crud) but sinks when you let off slightly(allowing slow leak-down to the internal RABS accumulator while the valve is held under lessor pressure), take the truck out on a dirt road and brake hard enough to lock up the rears so the RABS activates. Do this several times and with luck, the crud will clear as the dump valve is cycling and the fluid is flowing back and forth into and out of the accumulator. If that doesn't clear the crud, you will need to replace the RABS valve(you may have some luck with disassembling and cleaning it but not likely and very costly, time wise, considering you have to re-install and bleed it and then may have to R&R and bleed all over again, etc.) If you do anything, remember to flush all fluid thoroughly as you do not want any cleaned or new valves getting the same symptoms. Also remember to flush and replace fluid every two years to avoid contamination problems(yeah right). Here's a link with some good info: http://www.aa1car.com/library/abs_kelseyhayes_rwal.htm