M998 Brake Issues

Ok guys I have done some searching but have not really found anything specific to my problem. This is on a 1988 m998 I am purchasing. After an test drive and inspection the only issue I am finding is the following.

Brake light indicator light on dash stays on even with the park brake disengaged. You can pump the brakes and it will go off but them come back on.

Hand brake adjustment knob is adjusted all the way counter clockwise (to the right).

A hissing noise is heard near the brake pedal but pumping it will stop hissing but start back.

Brakes seem ok but since I am new to the whole HMMWV I am not sure what the brake pedal is supposed to feel like.

Any guidance would be greatly appreciated.

For now ignore the "hissing" unless you have fluid out of containment (leaks)
You should bleed the brakes and see if that helps the light issue, was it low on brake fluid?
BTW do not get too carried away on the pumping the brakes cuz you could hasten the demise of the seal in the hydo-boost, not a good thing.
The hissing could be the Cadillac valve directly above the brake pedal in the engine compartment or the hydro-boost itself, or something else, further investigation is required.

Mogman said:

For now ignore the "hissing" unless you have fluid out of containment (leaks)
You should bleed the brakes and see if that helps the light issue, was it low on brake fluid?
BTW do not get too carried away on the pumping the brakes cuz you could hasten the demise of the seal in the hydo-boost, not a good thing.
The hissing could be the Cadillac valve directly above the brake pedal in the engine compartment or the hydro-boost itself, or something else, further investigation is required.

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TYVM for the response. It was actually in the shop for brake work which is where I found it and asked about buying it. I am in talks with that mechanic but he is NOT well versed on the HMMWV and its brake system. He changed out the back drums for a sticking issue. I am texting him the info you discussed. I don't think he knew to use DOT 5 and probably used what he had on hand, most likely DOT 3. Would using the wrong fluid cause the light to come on like that?

Probably not if it had been bleed correctly, of course flushing the system with DOT 5 should be a priority.
If it were me I would flush it thoroughly drive it a while and flush it again, not going to be cheap, was a big mistake.

Master cylinder internally bypassing.

Suspect master cyl is what I would troubleshoot.

My light stayed on when I got it from auction and I checked a few things and it ended up being the emergency brake was over tightened and I adjusted it and light went out and hasn’t come back one

Maxjeep1 said:

My light stayed on when I got it from auction and I checked a few things and it ended up being the emergency brake was over tightened and I adjusted it and light went out and hasn’t come back one

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That is what I was thinking. Like I said the brake adjustment on the brake handle is turned all the way counter clockwise. Not sure it should be that way.

Unplug the switch at the proportioning valve, if the light goes away, you have bubbles in the system.

RJTM998 said:

TYVM for the response. It was actually in the shop for brake work which is where I found it and asked about buying it. I am in talks with that mechanic but he is NOT well versed on the HMMWV and its brake system. He changed out the back drums for a sticking issue. I am texting him the info you discussed. I don't think he knew to use DOT 5 and probably used what he had on hand, most likely DOT 3. Would using the wrong fluid cause the light to come on like that?

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Drums?

Wrong fluid?

That’d be enough for me to walk.

SmartDrug said:

Drums?

Wrong fluid?

That’d be enough for me to walk.

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Mis-spoke rear brake calipers. Still waiting on him to respond what fluid he used, I am just guessing because this is not what he usually works on. I don't think that is a good reason to walk because I think it is a nice example for what I am paying. I can get it bought for $12k.

Just a guess... Take a look at the proportion valve switch in the brake systems. Not much to see, But if front or rear hydro circuit is not correct it will turn on the light. Something to keep in mind. Located front of master cylinder off a bracket, Two wires to switch.

Not a fan of mixing fluids, IMO BIG no no. Emergency brake handle and switch is easy to see operation and figure out the light activation. Trouble flow charts are in the TM and a good time to reference. OUCH

Guessing on the web, CAMO

Agree with those saying to investigate the parking brake setup to address the Brake light indicator being on. That light is ONLY related to the parking brake.

You mention the mechanic said sticking rear brakes... Those too can be related to a mis-adjusted parking brake. Whenever rear brake rotors are replaced, the parking brake MUST be readjusted/recalibrated to eliminate any dragging of the parking brake. This dragging is hard to feel, but it is THERE. It sounds like you need to both recalibrate the parking brake at the rear rotors and tweak the parking brake light sensor itself.

Hissing is usually the steering/fan proportioning valve (some are made in Cadillac Michigan, hence the name stamped on them, though not all). There are plenty of threads on here about reducing the hiss, most involve wrapping the valve in a bit of inner tube or rubber to dampen the transference of vibrations to the body or surrounding air, which we hear as a hiss.

It's worth checking the fluids as noted by others, and DEFINITELY check that only ATF has been used in the steering pump. That is a very common mistake. HMMWVs use ATF for the transmission and the steering/cooling fan. If the mechanic or previous owner put steering fluid in, it's worth bleeding it all out and refilling (perform the bleed per the TM, to include several left-right stop-to-stop tire turns if you bled it dry).

Nothing you mentioned sounds bad to me. If it runs well it's probably OK to buy, but if the people owning/working on it weren't using the TM (they weren't by the sound of these issues) you should be ready to perform the entire lube order and standard annual maintenance, opening and inspecting the geared hubs and so on). All easy to do, just time-consuming.

Welcome to the Club!

Bulldogger

The light is NOT only associated with the parking brake, it is also connected to the proportioning valve by the master cylinder, the fact that it goes out when the brakes are pumped would seem to indicate an imbalance (possibly air) in the brake lines.
it is simple enough to check, just unplug the lead from the proportioning valve and see if the light goes out, if not then it is the parking brake, it would only take one minuet to figure out which is the culprit.

Also unless the 1988 M998 has had its parking brake updated then it has nothing to do with the rear calipers as the parking brake is on the back of the transfer case.
And if it has been updated (parking brake cables to the rear calipers) then you would need to drive back and forth several times applying the brakes to get the rear calipers set correctly, (also cycling the parking brake several times) this could also cause an imbalance in the brake lines.

I sure appreciate every ones input. When I get it bought I will report back on what I find. Thanks again for all the help!!

Mogman said:

The light is NOT only associated with the parking brake, it is also connected to the proportioning valve by the master cylinder, the fact that it goes out when the brakes are pumped would seem to indicate an imbalance (possibly air) in the brake lines.
it is simple enough to check, just unplug the lead from the proportioning valve and see if the light goes out, if not then it is the parking brake, it would only take one minuet to figure out which is the culprit.

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Part of this trouble shooting sequence would be to bleed the brakes. Just unplugging the switch will not remove air or indicate the master cyl is bypassing. Nothing special to this being a hmmwv in standard brake troubleshooting just like any vehichle. We are not working on the hydroboost side of the equation. Refilling and flushing can be part of the process as dot 3 and 5 have different properties and do not mix well.

Pumping the brakes and the light goes off indicates air in the lines or system internally bypassing. Other possible component failure is the portioning valve/switch but start with the basics.

If you buy this hmmwv, find a different mechanic !

juanprado said:

Part of this trouble shooting sequence would be to bleed the brakes. Just unplugging the switch will not remove air or indicate the master cyl is bypassing. Nothing special to this being a hmmwv in standard brake troubleshooting just like any vehichle. We are not working on the hydroboost side of the equation. Refilling and flushing can be part of the process as dot 3 and 5 have different properties and do not mix well.

Pumping the brakes and the light goes off indicates air in the lines or system internally bypassing. Other possible component failure is the portioning valve/switch but start with the basics.

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Removing the wire from the proportioning valve only confirms if it is an imbalance issue or a parking brake switch issue, no use chasing an imbalance issue if it is a parking brake switch issue.
If the light goes out which I believe it will just tells one where to go next.............
The VERY first thing I suggested was to bleed the brakes, and I put a high priority on flushing and refilling with DOT 5 as soon as he said it may have DOT 3 fluid in it.

I am going to be controversial here.

You are looking to buy a vehicle. The vehicle has damage and they have no idea what they're doing. Additionally they used DEX3/4.

I would do the following... Tell them that this is a very expensive issue and ask what he wants for the vehicle and then offer him $5000 less and then show him the prices of Wildwood brakes on ModMafia.

Then make an offer and if he doesn't accept it walk away. Trust me he will call you back.

If you are trying to figure out how to fix a vehicle I am happy to help you.

You are trying to buy a vehicle and that is a different animal, and my advice is based on trying to get you the best price you can get.

If and when you purchase the vehicle I and all the members here will be glad to work with you to give proper mechanical advice.

Now is the time for "the art of the deal". It is not the time to fix someone else's car so then you can pay a premium price.

Action said:

If you buy this hmmwv, find a different mechanic !

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Oh trust me I am. LOL Mostly will be done by me if at all possible. I would have tought he would have done a simple Google search

TOBASH said:

I am going to be controversial here.

You are looking to buy a vehicle. The vehicle has damage and they have no idea what they're doing. Additionally they used DEX3/4.

I would do the following... Tell them that this is a very expensive issue and ask what he wants for the vehicle and then offer him $5000 less and then show him the prices of Wildwood brakes on ModMafia.

Then make an offer and if he doesn't accept it walk away. Trust me he will call you back.

If you are trying to figure out how to fix a vehicle I am happy to help you.

You are trying to buy a vehicle and that is a different animal, and my advice is based on trying to get you the best price you can get.

If and when you purchase the vehicle I will be glad to work with the other members here to give you proper advice.

Now is the time for "the art of the deal". It is not the time to fix someone else's car so then you can pay a premium price.

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I do appreciate all the feed back and advice. First thing first. I feel based on my research I am already getting a fair deal and price has been negotiated. So we are clear on what is what on the purchase it is an 1988 (Motor Pool rebuilt in 2000) speedo showing 17k which I tend to think is correct but who really knows. I am thinking it was probably a National Guard Unit. It is an overall very clean soft top 4 man (with cargo cover) unmolested and all original. Only negatives that stand out is no doors, and brake issue that I have mentioned. No on road state title (has SF97) but NC is pretty easy to get one.

We negotiated it to $12k so I highly doubt he is going to take $7k for it. If you know of clean examples with some minor brake issues for less than $12k give me a PM.