HMMWV brakes - no pedal pressure at all, pedal to floor, then SPLAT!

I purchased a used 1994 M998A1 HMMWV from GovPlanet, got the engine running well this winter and was ready to take first spring test drive. Couldn't wait to finally drive it and enjoy the nice spring day. But I noticed that the brake pedal pretty much went to the floor without any resistance. Since this is my first HMMWV, I'm not familiar with how HMMWV brakes should feel but I thought this was odd.

I checked the brake fluid reservoir and found one of the two halves to be empty. I added fluid (DOT5) until the reservoir was about half full. I started the HMMWV again, still no pedal resistance. I pumped the pedal slowly 3 or 4 times, still no resistance at all, but suddenly SPLAT! I heard something that wasn't good, I got out and saw that the rear driver side caliper had a an explosive leak of brake fluid all over the ground , and continued to drip as the line drained out.

What would cause the lack of any brake pedal resistance? And yet after pumping the pedal a couple of times (without any resistance at all) it blew out something, maybe the hydraulic cylinder seal? Any suggestions or guidance would be great. It does not look like a fun job to work on the rear brakes of a HMMWV, this is new territory for me, I've only worked on civilian vehicles to this point and I'm looking forward to getting into funner toys!

I have the following style of master cylinder with the booster unit with the square flat metal lid - Brake Master Cylinder AM General 12342898

Air in the system = no pressure. Looks like you found your leak either way... I highly doubt you need a new master, replace, or if you can find a rebuild kit for the caliper... However, I would inspect it first. You may have blown a line... Anything is possible. ymmv

tage said:

Air in the system = no pressure. Looks like you found your leak either way... I highly doubt you need a new master, replace, or if you can find a rebuild kit for the caliper... However, I would inspect it first. You may have blown a line... Anything is possible. ymmv

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Ok, so maybe that explains why one-half of the master cylinder reservoir was empty... it had mostly leaked out before I got the vehicle. I'll check the lines. Are there any special tools or dissambly to get at the caliper's hydraulic cylinder ? And thank you very much for the feedback !

First off your going to need these
[h=1]TM's for the M998 series[/h]Brakes are covered in
TM-9-2320-280-20-2.pdf
Chapter 7

You need to do a full brake inspection and service where needed . Calipers should be replaced in pairs ,

porkysplace said:

First off your going to need these
TM's for the M998 series

Brakes are covered in
TM-9-2320-280-20-2.pdf
Chapter 7

You need to do a full brake inspection and service where needed . Calipers should be replaced in pairs ,

Click to expand...

Thank you porksplace, wealth of info there. I'll read through that manual, looks like there are great schematics throughout.

TheGreatMidwest said:

Ok, so maybe that explains why one-half of the master cylinder reservoir was empty...

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Simple. the res is split into two parts for the front calipers, and the rear calipers. with one side was drained confirms you have a leak in the rear brakes...
I bet now the system is dry for the rear. Will make the rear less messy on disassembly.

Remember to keep any pool shock you have laying around away from your leak...

tage said:

Simple. the res is split into two parts for the front calipers, and the rear calipers. with one side was drained confirms you have a leak in the rear brakes...
I bet now the system is dry for the rear. Will make the rear less messy on disassembly.

Remember to keep any pool shock you have laying around away from your leak...

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Thank you Colonel. True, I guess it will be easier to work on now that's it's dry. By keeping pool shock away are you referring to that chlorine powder put into swimming pools? Does that make for some interesting chemistry if pool shock mixes with brake fluid?

TheGreatMidwest said:

Thank you Colonel.

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No, that was my grandfather...

Just stick around, we're here to help.
It's a violent exothermic reaction with calcium hypochlorite actually.

tage said:

No, that was my grandfather...

Just stick around, we're here to help.
It's a violent exothermic reaction with calcium hypochlorite actually.

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Dangit, another distraction... now I've got to run a lab experiment before I get back to my brakes, you sparked my curiosity!

Once it went dry you will have to bleed the brakes again...the front Res is for the rear brakes, rear Res in the fronts.
plis fix any leaks that caused it .

I just replaced my rear brake pads, while i changed out the rear differential output seals. The fun part was reaching the 9/16" upper caliper bolt, and then rotating the piston with large vise grips while using the compression tool to push the piston back in. This is different to any brake job I had previously ever done.

In your case it sounds like you will be removing the calipers, which will make it easier by far.

Retiredwarhorses said:

Once it went dry you will have to bleed the brakes again...the front Res is for the rear brakes, rear Res in the fronts.
plis fix any leaks that caused it .

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Thanks for the added details, then that makes perfect sense, it was the front reservoir that was empty (back brake is bad), What would a project be without surprises. The willpower grows stronger! Must resurrect this beast.

Mike82ndABN said:

I just replaced my rear brake pads, while i changed out the rear differential output seals. The fun part was reaching the 9/16" upper caliper bolt, and then rotating the piston with large vise grips while using the compression tool to push the piston back in. This is different to any brake job I had previously ever done.

In your case it sounds like you will be removing the calipers, which will make it easier by far.

Click to expand...

Somehow you made me happy that I have caliper trouble. I'm glad it's only the calipers at this point, sounds like you needed an octopus to assist you with that output seal job!