very poor brakes

[russfixr]

The brakes on my '98 K3500 have always been fair. Last year the power steering pump started acting up and I replaced it. All has been well until today. Took the truck to the end of the drive and the pedal went to the floor. I was able to stop it by putting my leg into it - loss of boost? Not losing any fluid. Steering is fine so I doubt there is a problem with the pressure to the brake assist. Pedal also goes to the floor when the engine is off. I don't think this is normal, but not sure.

Is it a loss of the brake assist unit behind the master cylinder, loss of the master cylinder or a problem with the antilock braking? What checks are there?

It's fair week so I have to get this thing back on the road fast or deal with an unhappy daughter and wife - help!

Thanks,
russfixr

[JoeyD]

I would guess master cylinder is bad. But try pulling the ABS fuse and see if it changes any, may be a faulty unit but I would bet it's the master.

[Robyn]

The master cylinder has gone south.
Get a fresh one and pre bleed it on the bench so you dont have any air in it.
Once you install it loosely tighten the fittings and then have an assistant push the pedal while you crack the fittings at the cylinder to allow any trapped air at the fittings to ascape.
You should be good to go from there.

Good luck and keep us posted

Robyn

[tommac95]

My experience (ongoing observations):
If the pedal oozes down when engine compartment good and warmed up [also , the system brakes finely/firmly/stably when the underhood is cool/cold] , and the 'brake' panel light illuminates , that's internal leakage in the MC , as from wear, debris, cut rubbers , etc.

If the pedal goes low only when the engine is running at low rpm/idle , and the 'brake' warning light doesn't illuminate , and this occurs both cold & hot ... i think this is a result of a worn/spent hydraulic accumulator in the hydraboost assy.

However , i'm also afraid that debris from filthy remanufactured powersteering pump assemblies may be screwing up the works ... either the hydraboost , or the ABS module valve components....

It helps to occassionally adjust the rear drums....

[russfixr]

Thanks for the input. I replaced the master cylinder the other night. Exact same symptoms. I just got done replacing the hydraboost assembly. Brakes respond much better. Brake light won't clear. Should it have cleared on it's own or is there a way to clear it?

Thanks,
russfixr

[tommac95]

Russ
OK, did you follow Robyn's advice and pre-bleed the MC on the bench before install?
On my old MC sometimes the light illuminated when the pedal dropped wayyyy low ... after allowing the pedal to rise back up , if i gave it a good 'pump' and release , the light would extinguish {think the piston was returning to its proper position}. Mine is OBDI, your results may vary.

If you did not bench bleed , there may be some trapped air in system, may need a pro/power bleed.

Be advised there are special hydraulic system codes in the onboard diagnostics , so a scan may be of some assistance.

Again , possibly a ABS complication???

I'd like to have DMax chime in....

[DmaxMaverick]

Looks like you guys? got the bases covered. Stay after the MC. You must bleed it properly, or the pedal will be like stepping on a plum. Don't count out a bad MC right out of the box, either. Seen too many of those. Probably in the neighborhood of 1 in 4 bad (mostly Cardone, and their off-brands). There seems to be a problem getting good castings these days, and it's not limited to MC's. I suspect the "country of origin" may have something to do with it.

That said, the ABS controller could be, although very remotely, the problem. When they fail, they default to "no action". But, if an internal valve is worn, corroded, or otherwise failed open, it could be bypassing. I doubt this is the case, as it would surely trigger the ABS lamp. Pulling the fuse will confirm the lamp works.

If the brake warning lamp is on, you have a pressure issue. Either the MC is leaking internally, air in the system, or a proportioning valve is stuck or worn out. A worn out valve would not cause a soft pedal. If the MC is good and bled properly, you should have a stiff pedal, regardless of what the hydroboost is doing. A failed accumulator will effect engine off reserve braking, but should have no effect with the engine running. All this assuming you have no external leaks and are not losing brake or PS fluid.

[russfixr]

I did a bench bleed on the MC but I think it is an air issue. Stops pretty good once the pedal is half down and the pedal is slow to come back up. Tried to bleed the brakes and uncovered a whole other can of worms. Bleed fittings on the front calipers broke even after two days of penetrating oil, a little heat and rapping them once or twice to try and break rust loose. Pulled the drums off the back and everything was covered with 80W90 due to bad seals. I'm pretty much going to have a new brake system when this is over. New back pads and might as well do the cylinders while everything's opened up, new front calipers. Front discs and rotors only have about 5K on them.

I got all the parts and just need more than 2 hours to put everything together and finish a complete bleed.

Thanks for all the help, I'll let you know how things come out.

russfixr

[russfixr]

The brake issue is resolved. They are actually better than they have ever been. Once I got all the previously mentioned parts replaced and did a very extensive bleed, I had a good hard pedal and nicley reactive brakes. I would say the keys would be well adjusted back brakes and a good bleed. I used the Motive Products power bleeder that was mentioned in a previous thread. It made a substantial difference over the the manual bleed I did before my bleeder arrived. Thanks for all the advice, it really proved invaluable.

russfixr

[Robyn]

Glad to hear that all is well again

Robyn