Fixing the 1998 HMMWV

Hydro Boost Unit - Removal

I noticed the driver's side toe board / firewall was wet back in February, when I was replacing the brake switch. It's pretty obvious that the hydro boost unit was leaking. That, and also the fact that it didn't work.

IIRC, if the hydro boost unit is leaking on the engine side, it's repairable, as it's probably just a seal; leaking inside the cabin means the unit is un-repairable, as it's from the pushrod. In this case, I don't believe it's repairable at my level, but I'll let others chime in if I'm wrong.


For this one, I'm going to need to get out some new brushes for scrubbing.


I'm also going to need a set of metric flare nut wrenches.


The hydro boost unit bolts directly to the firewall, sitting behind the brake master cylinder. The first step then, is to get the brake master cylinder out of the way.


The master cylinder only has two nuts holding it onto studs at the hydro boost unit; one is on the left (outboard) and one on the right (inboard). Both have some hardware attached, so it will take a few extra steps to remove.

First, this bracket goes from the master cylinder to the left splash shield and will need to be removed, as the nut you need to access is underneath. In my case, I had already removed the splash shield, so this was no big deal.


15mm seems like a good place to start.


With the splash shield bracket removed, the outboard (left) master brake cylinder nut can be removed from the stud on the hydro boost unit.


Second, on the inner (right) side, remove the nut and washer holding the proportioning valve in place. The nut you need to access to remove the master cylinder is also underneath on this other side.


Once it's loose, gently move the proportioning valve out of the way. In my case, it didn't need to be disconnected.


With the proportioning valve out of the way, the inboard (right) master cylinder nut can be removed from its stud on the hydro boost unit.

Once loose, the entire master cylinder can be moved to the side. In my case, there was plenty of length in the brake lines to do this without pinching or bending anything. Since we didn't remove any brake lines, the brake system shouldn't need to be bled afterwards either. Added bonus.


Keep track of the hoses, and which ones are supposed to go where. I'm replacing these hoses at this same time, so I'm not super-concerned with dirt, but otherwise I would be.


There should be two high-pressure hoses, and one low pressure return hose. Of the high-pressure hoses, the outboard one goes to the steering gear; the inboard one, and the low-pressure hose, both go to the power steering pump.

I damaged the low-pressure hose getting it off, but again, I'm replacing all of these.


A 19mm flare nut wrench will get the high-pressure hoses off without any damage.


Both high-pressure hoses will also have O-rings that will need to be replaced.


Finally, there is a small drain tube at the bottom of the hydro boost unit. This isn't shown in the TM -20; this is actually a leftover group 3305 part for the DWF kit. Just remember it's there before yanking out the unit.


Here's the same picture, only cleaner with better lighting.


Inside the cabin, there are 4 nuts, with lock washers and flat washers, to remove the hydro boost unit.

The hydro boost pushrod is connected to the pedal with two washers and cotter pin. The outer washer is a 1/2" flat washer, while the inner washer is a 1/2" spring tension (wave) washer. I'm not quite sure at the moment how the later will be replaced.


A long socket extension with a universal joint makes this pretty easy.


I don't remember what this picture was for, but obviously the entire unit is disconnected; make sure the hydro boost unit doesn't fall out once the fasteners are removed. You might want someone to hold it on the opposite side.


Again, don't forget to remove the drain hose.


Here's the old unit. Still leaking, even while it's sitting on a table...


Here's the new one.


We'll save what happens next for another post, probably later today.

I had originally tried to enlist some help and moral support with BBQ, but didn't get any takers.


How about a seafood boil? There were more shrimp, but the kids got 'em. The "slap" level was only medium, but Momma said it was too spicy this time. This is what's still left after days of eating. I can always make more.

I decided this morning to take out the cooling stack before going any further. This needed to happen regardless, and if I'm starting to replace hoses, it would just be easier with everything out of the way. I'll come back to the hydro boost unit in just a while.

The unbolting of the power steering cooler was in an earlier post, so after taking two loosely held, previously stripped, bolts off, I was done. The transmission and oil cooler took a little bit longer.

For the oil cooler side, the largest wrench I have is a 24mm, which wasn't going to be enough. Fortunately, a pair of crescent wrenches did the trick.


The ends on the oil cooler side don't turn, so you'll break something if you try too hard. Hold the fitting on the cooler side. Turn the fitting on the hose side.


It will drip oil. So, keep some rags or whatever handy.


The other side of the cooler is for the transmission fluid. This is just 5/8" low-pressure hose with some clamps. Nothing special.

A box of plugs and caps from Harbor Freight is useful here.


The Harbor Freight kit has plenty of types and sizes. I used either the grey or white plugs, and either the green or blue caps, depending on the line size. My eyes are too old to read the included chart; I just used the ones that fit best.






Power steering cooler is out; transmission and oil cooler are now out; last to go is the radiator. Thankfully, there were no more stripped bolts to deal with. As an added bonus, I also found two wrenches I had been missing.

mrandig said:

I wasn't planning on writing about radiator combs this morning, but here we are.

This first item is actually an alternative to radiator combs. This works for me; your results may vary. Radiator combs don't work great if there's heavy damage.

Here's a plastic tool for interior trim. I don't remember how expensive it was, but I don't think it was very much. I know I bought a full set of these back in the day at O'Rielly's Auto for maybe $25?
View attachment 943305

The nice thing about this tool is that it is tapered and longer than most combs from Amazon or the local parts place. It only does one row at a time, but it does a good job. Obviously don't use the wide face; use the narrow side.

Below are some pics of today's battle with my transmission cooler. It was beat like it had owed someone money at some point. I'm not done yet, and there's still some additional straightening needed in spots, but good enough for now. There are some areas here that are so bad they're not going to be fixable.

Before:
View attachment 943306

After:
View attachment 943307

Before:
View attachment 943309

After:
View attachment 943310

Full view after <1 hour's work:
View attachment 943311

The blades may be straightened at this point, but unless you've got a steady hand and a good eye, they likely won't be perfectly parallel. Now if you wanted to go a step further and get a fin comb involved, it's a lot easier.
View attachment 943788

The correct comb is #10, at least it is for my HMMWV. This number refers to 10 fins per inch.

Depending on the brand, I've seen this particular type go for anywhere between $10-$25.
View attachment 943791

With a comb, things are starting to look even better; all of the fins are straight and aligned.
View attachment 943789

This method also works great on the power steering cooler. In this case, there is some damage on the lower part that I can't fix, because the fins are too far gone. By "gone" I mean they're torn out.
View attachment 943790

The radiator should be getting replaced sometime over the next few days, so I'm not so worried about that.

Until next time, friends!

Click to expand...

I was rambling on a few weeks ago about radiator combs, and cooling fins being straight, etc. I'm sure at least a few people stopped reading this thread at that point.

Here's where the story comes together.

Essential tools.


I was also using an automotive trim tool, but I was too lazy to find it for this picture.

Paint prep and specialty radiator paint.


Almost ready for refinishing.


Power steering cooler is done.


It's hard to see, but it does look amazing. Perfectly straight, totally clean, just like new in satin black.

Transmission and oil cooler is next up.

Transmission and oil cooler completed.

Now I just have to find somewhere to keep everything safe until it's time to put it all back together.

If I was going to make a post on how to remove the radiator in a M1123, I missed my opportunity. Once I got rolling, any chance of taking pictures went out the window.

Let's just say it takes at least two people and will probably involve a bit of cussing.

At least it's out.

Yea that radiator isn't a lot of fun in my experience either. But like you said, it's done.

Now that I'm into the guts of this thing, I think I'm going to need a bigger parts order. There are more and more things that should be replaced just while I'm in the neighborhood.

I'm also going to need some spray paint for touch-ups. I'm assuming the paint on the frame is the same CARC as the body. I will need to look around the site, as I've seen this information a few different times; I'm just too tired to do it tonight.