Suggest next steps for brake troubleshooting (booster replacement I think?) (1 Viewer)

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I've never loved my brakes, but they were working a lot better until just the past few weeks when the truck sat after our New Year's trip. I jumped in last week and noticed the pedal sinks nearly to the floor and the engagement isn't very confidence inspiring, hard to panic stop. In a rush to make them work better before a trip up to the (snowy) mountains yesterday, I bled the LPSV and activated the ABS a bunch, which didn't change anything. I've been through most of the brake system in the past 6-12 months (new rotors/calipers/pads front and new pads rear, plus a lot of bleeding) and as recently as three weeks ago the brakes were working fairly well.

In testing, I think maybe my brake booster has failed as I'm not getting any pedal travel down when holding the brake pedal and starting the engine--only hesitation before I spend the $300 for a new booster is that most of the troubleshooting guides (chart in FSM, plus threads here) suggest a failed booster should cause a hard pedal, not a soggy one. Is there any other logical next step you guys can recommend or should I just bite the bullet and buy a new booster? I can pump up the brakes with the engine off in about 3 pumps and then hold it there, so I think that suggests my MC is OK, but I've not touched either the booster or the MC in my time with the truck (18 months.)

Thanks for any other ideas.
 

richardlillard1

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Brake booster failure would lead to a hard pedal. A sinking pedal, would be indicative of a leak in the system. If there are not visible leaks, I would turn my attention to the master cylinder, as the seals within the master are capable of failure, leading to a soft pedal from the reduced pressure.

This is assuming there is no air in the system.
 
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Brake master cylinder. I had the same issue on my 1997 years ago. Works most of the time then suddenly in traffic the pedal sank to the floor then I release and reapplied pressure for it to brake. Removed the master cylinder to rebuild and found debris inside.
 

mudgudgeon

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How are the front wheel bearings?

My brake pedal recent went spongy, would sink 3/4 to the floor. Brake were still working, just not confidence inspiring.
But sitting still with the engine running, the pedal was firm after a second pump.

After a quick look over the brakes, clean fluid, no sign of leaks, I decided it was front wheel bearings.

Both sets of wheel bearings were noticeably loose when I checked them.

Retorqued the bearings, and the brakes are 100% back to normal.
 
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I just went through this with my buggy, which has a very simple brake system. No LSPV and no ABS. You need to go through each wheel before you doing anything else. Make sure the front pistons retract, make sure the slide pins on the rear are in good shape, check pad thickness. After that go to the master, You can test it on a bench or in the tuck by removing and plugging the brake line hose outlet and see if it holds pressure. If you remove the master you can do a visual check on the booster. See if the rod has play in it. I ended up replacing brake pads, replacing two calipers, replacing master cyclinder twice, a new one was bad out of the box, and replacing the booster. Also, I found the best way to bleed the system is with a pressure bleeder from the master. I was able to borrow one from a friend. Makes bleeding the brakes a one man job.
 
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Thanks for the replies all. I ordered an Aisin MC and some rear calipers as I noticed last night that my passenger rear has a dragging piston. Will crack into this weekend/next week once parts arrive. Worst case I guess is I get all of that in, the brakes are still "meh" and then I'll know it's probably brake booster related.

Appreciate the ideas.
 
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Just updating. New master cylinder in, new rear calipers and rotors, bled the lines (five locations), activated ABS a bunch (with truck up on four jackstands) and then re-bled the lines...problem persists. Soft pedal sinking further than it should with not enough brake power to lockup on pavement.

Going to crack into the front next, maybe the wheel bearing suggestion is the right call although I don't notice excess play when shaking the wheel with it in the air...Any other ideas are welcome.

Cheers.
 
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You can adjust the pedal position. There’s a threaded shaft that connect the pedal to brake booster. You extend it at 1/4 to 1/2 turns.
 
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Thanks. I saw that but couldn’t make sense of the FSM and the SST pictured to see if mine was in the right range. This is what you're talking about right?

1612310088612.png
 

richardlillard1

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You can adjust the pedal position. There’s a threaded shaft that connect the pedal to brake booster. You extend it at 1/4 to 1/2 turns.
The pin adjustment on the booster will either make the brakes come on (if adjusted out too far) and leave the brakes actuated on a constant basis, or not provide boost to the brakes, leading to a firm pedal with no assist.

Once again, the booster is for aiding the brakes. Any issues in that system will not cause a soft or sinking pedal. The problem is elsewhere in the system.
 
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Yeah, understood, and thanks...I was hoping it was my master cylinder...the investigation continues...
Don't be so quick to think that. One problem could be that a seal inside the booster has given way, allowing air to be introduced into the system and have a soft pedal. Any sort of air woosh sound exist when you push the pedal with the engine off?
 

jbee

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I put an OEM booster in my current ride, took less than an hour including a little bit of wrassling to get it out. Partsgeek and cruiseryard both carry that OEM style seiken one.
I put an aftermarket one in my old 96 cruiser and it was fine, I think the Cardone and Centric stuff is fine. You’d be under $200.

I would not be man enough, however, to try the sub-$100 Amazon or eBay ones.


 

flintknapper

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Don't be so quick to think that. One problem could be that a seal inside the booster has given way, allowing air to be introduced into the system and have a soft pedal. Any sort of air woosh sound exist when you push the pedal with the engine off?

Not likely booster related from the sounds of it. Booster works off of engine vacuum. IF the diaphragm inside the booster were compromised....OP would definitely experience a hard pedal. IF the seal(s) in the master cylinder are leaking then brake fluid gets sucked into the booster, not air into the master cylinder usually.
 
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Not likely booster related from the sounds of it. Booster works off of engine vacuum. IF the diaphragm inside the booster were compromised....OP would definitely experience a hard pedal. IF the seal(s) in the master cylinder are leaking then brake fluid gets sucked into the booster, not air into the master cylinder usually.
That's what I thought at one point. Had a friend with an audi have the pedal go soft on him driving (how dare I bring about non Yota names here!). Was absolutely convinced it was the master, which a new one did nothing to solve the problem. Ended up being the contaminated brake fluid got into the booster and eat away at it. We never experienced the hard pedal condition that is always described with a bad booster. Not to disagree with you by any means, just in my experience for this scenario you can't rule it out quite yet. When we took a test drive around the block with the new master resulted in almost no brake action until almost the floor.
 

flintknapper

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That's what I thought at one point. Had a friend with an audi have the pedal go soft on him driving (how dare I bring about non Yota names here!). Was absolutely convinced it was the master, which a new one did nothing to solve the problem. Ended up being the contaminated brake fluid got into the booster and eat away at it. We never experienced the hard pedal condition that is always described with a bad booster. Not to disagree with you by any means, just in my experience for this scenario you can't rule it out quite yet. When we took a test drive around the block with the new master resulted in almost no brake action until almost the floor.
^^^^

We are in agreement, it's 'possible'. Very odd and rare failure type, but yes....can happen.
 

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