I have seen the question asked a million times on other forums. It usually comes down to someone claiming to be an awesome mechanic stating that hydroboost brake boosters aren't rebuild-able/resealable or that they are so complicated if you took them apart you would never get them back together.
Well even though my hydroboost has been doing a great job keeping my undercarriage lubricated the constant top offs with power steering fluid and air in the system got pretty annoying. I decided that I would take a peek inside of it and possibly reseal it. I didn't plan to rebuild the unit as there are a couple specialized tools you either have to purchase or improvise.
First off when my power steering system wasn't low and had set for a bit to allow the air to work itself out the brakes worked fine. Once the booster saw air it was hard to push them and they had a tendency to jerk. So in other words my booster was functioning. Just leaking fluid and drawing air.
I started off by finding a kit. I purchased a MBM #50 kit that cost me $25 through piratejack brakes.
Next I flushed the system, not that it needed it because it went through powersteering fluid constantly.
Removing the brake booster is frustrating because of course it requires you to remove 4 nuts and the pedal rod from under the dash. The pedal rod is held on by a washer and clip. The 3 out of the 4 nuts were quickly removed using a 3/8 air ratchet, swivel, extensions, and a 15mm deep socket. The last one was removed using a ratchet, short extension, and a deep 15mm socket.
Of course there are 2 15mm holding the master cylinder to the booster. Two pipe fittings 16mm (power steering), and a hose that is held on by a clamp (also power steering).
Afterwards I had to remove two plastic line holders from the brake lines to allow them to move a bit. Then I merely slid the master cylinder off it's studs and moved it to the side being careful not to bend the brake lines. I found this gave me the benefit of not having to bleed the brakes and bench bleed the master.
At this point carefully manipulated the booster out of the truck.
Once on the floor I placed it on a rubbermaid container lid to catch any fluid that spilled out once opened. I removed 2 15mm nuts that held on the adapter plate that bolts to the master cylinder. At this point a star retainer, spring, rod, and plastic washer will come out. The star retainer will come off by itself but the spring, rod, and plastic washer assembly should remain as one piece.
Next I removed the 5 3/8" bolts that held the casings together. It took very little persuasion to separate the case halves. It is important to note at this point I remembered I had forgot to apply the brakes 10 times with the engine off to de-pressurize the booster. Not such a big deal cause I had no intentions of removing the gas canister on the booster anyway. Once the halves began to come apart the spool valve fell out. Not a big deal as it can only go back in one way. Oddly enough I found the check valve was missing and in the back of the casing. I noticed matching marks on the check valve and housing right behind the fork on the pedal rod. I am still not sure what happened here, cause the booster seemed to work. All parts were accounted for so I moved on to resealing it.
I removed the original figure eight seal that sits between the two housings. Comparing the new and the old as well as looking at marks on the housing it would seem this was my problem seal. The old seal was considerably thinner, probably due to shrinking. The new seal can be pushed into place with fingers.
Next I used some small hooks that I had to fish the piston seal out of the piston bore without damaging the bore. Pay attention to the orientation of the piston seal when removing it. Once again the new seal can be installed simply by fishing it in there and working it in with your fingers.
I now reinstalled the check valve and the spool valve. I connected the pedal rod to the spool valve and reinserted the piston into it's bore.
Just as before press the housings back together, this shouldn't take anymore than hand force. Reinstall the 5 3/8" bolts and snug them down. Back on the other side of things reinsert the spring, rod, and washer assembly. Place the star retainer back on the rod making sure to have the curved points toward the inside.
Reinstall the master cylinder adapter over this, it will retain the spring assembly. Tighten the 15mm nuts.
After that it is pretty much reverse of removal. Place the booster back in the truck. Tighten the 4 15mm nuts under the dash that retain it to the firewall. Reinstall the pedal rod, washer, and clip. Back under the hood reconnect the power steering lines, and master cylinder.
Top off the system with power steering fluid and work the wheel back and forth, lock to lock while pressing the brake pedal. It will feel funny at first but it will tighten up as the fluid gets to it. You may hear strange noises due to the air in the system but don't worry the air will work it's way out after sitting for a bit.
I have yet to notice a leak. Took her on a trip around the neighborhood for a bit after degreasing everything and haven't noticed anything new. My brakes even feel stronger. Hopefully this helps someone not spend $200 over a simple leak.