M998 Brake Issue

[Abrams66]

My 998 just delivered and ran/drove fine. Next day, had foot on brake pedal and it went all the way to the floor. No leaks in system at master or wheels and brake lines. Checked fluid levels of power steering pump and all levels good, no leaks. I ordered a new mater cylinder to be on the safe side. Any other thoughts as to root cause for brake loss?

 

[frank8003]

4 second search here, yep, 4
Maybe not what you want but the direction of the search is good...............
https://www.steelsoldiers.com/showt...dal-pressure-at-all-pedal-to-floor-then-SPLAT!

 

[Abrams66]

Thx...the link to the “half full master reservoir” was interesting, but my system is FULL + no leaks. Given this + power steering pump functioning and at proper fluid level, my guess is “replace master cylinder” and bleed system. Also don’t assume system is Dot3 or Dot5...I will sample and do the water + eye dropper of fluid mix test before beginning brake job. Does this sound like I’m headed down the right path?

 

[Action]

There would be no fluid near the wheels. The brakes don't go that far. The hydroboost could be bad? Try just bleeding the brakes. The system might be sucking in air but not pushing fluid out.

 

[86humv]

Make sure you buy the right M.C. , as there are 3 types.

 

[juanprado]

I do not have first hand knowledge or tested HMMWV brakes yet but in another lifetime sold parts.

The hydroboost is the mechanical assist in place of a vacum brake booster in gas powered engines to make the brake effort/pedal softer/easier. If the hydroboost was out, on the brake side you would have a rock solid pedal requiring more effort to travel.

A Master cyl that has no external leaks in the system but is to the floor indicates it is internally bypassing and the seals / cups on the piston rod have seen a better day. In the ole days, they would be rebuilt but it got to the point that re-builders could offer a product just as cheap as the master cyl rebuild kit.

my .02 would be to bleed but strongly think mast cyl is toast and needs rebuild or replacement.
YMMV

 

[papakb]

Take a look at the E place and compare the pictures there with what you have. You might also take a look at your cylinder for casting numbers and then head down to a good auto parts place like Napa and see if they can cross reference it for you. It's probably just a standard, off the shelf cylinder.

 

[juanprado]

Take a look at the E place and compare the pictures there with what you have. You might also take a look at your cylinder for casting numbers and then head down to a good auto parts place like Napa and see if they can cross reference it for you. It's probably just a standard, off the shelf cylinder.

Yes, I have not researched the casting numbers but looks like a very common 70's-80's GM 1/2 ton pick up mst cyl with power brakes before they went to alum bores with plastic reservoirs.

 

[86humv]

MC is either oval, or rectangle....not interchangeable.
I have new ones avail.