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Hydroboost repair

Mogman

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Well the "step on brake hard" part of the power steering pump test was too much for the 28 year old piston seal in the Hydroboost unit on my M998 and it started leaking between the master cylinder and the firewall.
A quick look around and the replacement cost was too much for my cheap a$$ so I decided to repair it myself, I had a pretty good idea what I would find even though this was the first hydroboost I have repaired, there would be an input rod attached to a piston and a biasing device that would be attached to a valve, the piston would push on the output rod to the master cylinder, but it so convenient to go on the interweb and U tube and look so I did, the problem with U tube is a very high percentage of what is there is WRONG, and every video I watched was wrong so I will post a couple picks here to show the correct way to replace the piston seal

I will not go into removal or disassembly of the hydroboost, I will just pick up where the train leaves the track,,,,,

And of course follow these instructions at your own risk, this repair cost me $11.00

There are some variations in the videos, some show how to remove it, some just start with the disassembling stage, but ALL pretty much say "replace the seal, shove it back together and go skipping down the yellow brick road, again WRONG.

The booster piston has a lip seal, you can SOMETIMES get away with shoving the piston against the seal lips IF the piston has a long camfered edge, mine did not.
SO first after replacing the seal with the seal lip facing the piston and the new seal ring between the two halves, leave these parts out.
Leave out.jpg
You can work the retainer (star) out of the "front" booster half with a pair of needle nose pliers, set up the "rear" half in a vice as shown here.
Linup.jpg
Make sure the valve is in the fork like this.
Fork.jpg
There is a spring for the valve that should be held in the front half with a glob of "assembly goo" or bearing grease, there is also a small dump valve in the front half that should stay in place unless you bang the front half around too much.
Now as you guide the front half over the rear half you will be looking down the front half where the master cylinder attaches and will see the piston as it meets the seal.
View.jpg
You will now take a lip seal tool like one of these, from left to right, loop type, spring steel type, my FAVORITE a dentist spatula, and far right is a seal pick for removing the old lip seal.
Tools.jpg
You will gently reach around the edge of the piston as it meets the seal and guide the seal into place, when it is completely in place the front half will fall by its own weight, do NOT force anything here, when the two halves are about 1/2" apart you will start to go against spring pressure so put the bolts in and tighten evenly.
Here is a rather poor picture of what can happen if you just shove them together, from about 1 o'clock to about 3 o-clock the lip is starting to turn backwards, it the piston was to be shoved any farther the seal would be cut.
lip.jpg
So now that you have the two halves together and torqued line up the output shaft as shown here with a socket to push the shaft.spring and retainer into the housing (note the retainer is turned around backwards in this picture)
Socket.jpg
You will have to push it in skewed a little bit, meaning some of the fingers of the retainer will be in place before the others, then the assembly will look like this, to be continued
 
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Mogman

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So this is what it should look like when complete...
Final.jpg

These are the only parts I used, of the many "universal" kits out there this uses the smaller dia. seal as it has the smaller dia. piston
Parts.jpg
If the booster is leaking into the passenger compartment it cannot be repaired as the brake pedal input rod is staked to the input shaft so the input seals cannot be replaced.
 
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