Brake booster gone bad?

[bluedge8]

It's hard to tell from your description whats going on. You have to completely different hydraulic systems in play here that technically don't connect to each other but are used together to complete your brake system. The power steering pump on your truck provides pressure to the steering gear and the hydraulic brake booster behind your master cylinder. It seems to me that one of 2 things is happening:
1: The power steering pump is not building or maintaining enough pressure to run both the steering and the booster at the same time, or the brake system is using way more pressure that it is supposed to to stop the truck.

or,
2: The master cylinder is not working correctly and is requiring way more pressure from the p/s pump to stop the truck which is bleeding off too much pressure from the p/s system and you can't turn the truck very well.

What I don't know is how to diag one vs the other and separate out the problems. I know brakes much better than hydro-boost so, In my opinion your brake pedal should not ever be able to go to the floor if the brakes are in good working order, so if it was mine I would work on the brakes first and the p/s system second, hope that helps.

 

[Arisley]

Probly master cylinder

the booster is just the symptom, you are using so much boost to try and stop the truck that it is using all of your pressure from the pump. You have a bad master cyl, it is bypassing internally. Can you step on the pedal to where it feels like a good amount of pressure, then hold that same pressure and see if the pedal drops within a second or so?

It's hard to tell from your description whats going on. You have to completely different hydraulic systems in play here that technically don't connect to each other but are used together to complete your brake system. The power steering pump on your truck provides pressure to the steering gear and the hydraulic brake booster behind your master cylinder. It seems to me that one of 2 things is happening:
1: The power steering pump is not building or maintaining enough pressure to run both the steering and the booster at the same time, or the brake system is using way more pressure that it is supposed to to stop the truck.

or,
2: The master cylinder is not working correctly and is requiring way more pressure from the p/s pump to stop the truck which is bleeding off too much pressure from the p/s system and you can't turn the truck very well.

What I don't know is how to diag one vs the other and separate out the problems. I know brakes much better than hydro-boost so, In my opinion your brake pedal should not ever be able to go to the floor if the brakes are in good working order, so if it was mine I would work on the brakes first and the p/s system second, hope that helps.

If you can't bleed air out of the system you have a bad master cylinder.

 

[TyCorr]

Can you explain that out a little farther Andy? If you would please!

 

[Arisley]

Spongy pedal is air, or valves in the master cylinder getting blow by. If there is no air in the lines from the master cylinder to the brake calipers, it must be in the master cylinder itself. Unless you ran the master cylinder out of brake fluid, you have a bad master cylinder. You can try doing a bench bleed on the master cylinder, but, once again, unless you ran it empty, it is not going to work. Valves in the master cylinder are bleeding.

Years ago, you could buy a master cylinder rebuild kit. I have not seen one in at least 20 years.

 

[GO BIG OR GO HOME]

Sorry I havn't updated this till now. The truck need a brake job regardless of the soft pedal so went ahead and got that out of the way. New pads, slide pins, and freshly turned rotors all the way around. Swapped ps pump for the 3rd time at oreillys and flushed hydroboost system again with synthetic fluid. Made a big noticeable improvement in pedal feel and brake performance but still wasn't stopping in quite the distance it should and pedal was still a little bit soft. Put a new matter cylinder on and it made everything work great.

I appreciate all the help and info from you guys. This was a good learning experience. Thanks!!! :thumbup:

sent while doing burnouts somewhere in the country

 

[Arisley]

Glad you got it fixed.

 

[TyCorr]

Glad you got it fixed.

Thanks for explaining that. I honestly have never had one go bad. I had an old.bread truck that I used for the pool.company that I replaced one on but Im not sure it was bad, drums on all four corners will lead you to.think your brakes suck.

 

[Arisley]

Drums on all four corners, your brakes do suck.

 

[TyCorr]

Thats the same nail I was swinging at,lol.

 

[GO BIG OR GO HOME]

So glad this one had rotors

sent while doing burnouts somewhere in the country

 

[lariat 7.3]

my truck has been doing the same thing since i bought it used in 2010, very spongy pedal. Replaced the MC couple years ago and no change. Thinking about just buying a new motocraft one. Even tried service bleeding the ABS with my AE. Still no change. Brakes absolutely are terrible. Go right to the floor with the rubber ball feeling

 

[bluedge8]

one other thing to check is the rubber hoses, once in a while they fail and won't hold pressure but also don't leak.

 

[lariat 7.3]

should i just throw a master cylinder in it?

 

[Arisley]

1. Start your own thread instead of piggybacking. It makes it a lot less complicated keeping up.

2. Did you bench bleed your master cylinder before installing it?

 

[lariat 7.3]

sorry, since you were all commenting i figured what the heck. And yeah i did. Its been about a year and a half or two years since i put that new one in and it didnt change a thing. Service bled the abs thinking it had air in it too last week and didnt help.

 

[Arisley]

No problem. Try doing a full gravity bleed of the entire system. Open the bleeders on all four wheels. Drain through clear hose into suitable containers. Remove the top of the master cylinder. Engine off. Keep the reservoir full, go through at least a quart. Sounds like you possibly have a big air bubble in the system somewhere. Check for any leaks, even a very minor leak in the brake system will allow air in the system.

 

[GO BIG OR GO HOME]

No problem. Try doing a full gravity bleed of the entire system. Open the bleeders on all four wheels. Drain through clear hose into suitable containers. Remove the top of the master cylinder. Engine off. Keep the reservoir full, go through at least a quart. Sounds like you possibly have a big air bubble in the system somewhere. Check for any leaks, even a very minor leak in the brake system will allow air in the system.

This is a great option and worked for me too. After my latest ordeal I ordered a vacuum bleeder from oreilly and it works like a champ. Was around 50 bucks and is one of the best tools I've ever bought

sent while doing burnouts somewhere in the country

 

[lariat 7.3]

yeah i did a gravity bleed a few weeks, and it made them more firm but not great. Then went back to the bad pedal feeling again. Almost like air is getting into the system somewhere. I went to do another bleed and wasnt paying attention and the resivoir may have run empty so i prob have a ton of air left. Bled at least a quart and a half through the system though

 

[TyCorr]

Check the fitting on the side of the PS pump. That goofy squiggly line sucks air like crazy and the intercooler piping beats on it. It could be loose and you wont even know. I have my original pump still so maybe its an obsolete design.

 

[lariat 7.3]

ive noticed it moving on mine, so that directly relates with the brake fluid?