Unless there is a foreign object that has lodged itself into one of the ports, it sounds to me like the control valve inside of the brake unit has hung up in the bore that it slides in. There is an exploded view on this page:
http://www.hydratechbraking.com/Hydratech/tech.html What I'm referring to is called the spool valve, which is actuated by the spool valve lever. The spool valve is about the diameter of a nickle and about 3" long - the clearance to the bore it operates in is half a thou, so any dust or particulate can hang it up.
I recall years ago we did have a customer send a brake unit back in to us to have it examined for this type of problem, and we did find a foreign black rubber port plug / vacuum cap had been forced with hydraulic pressure into the 18mm adapter on the driver's side of the brake unit in such a way to plug it solid. Please remove everything from the 16mm and 18mm ports and examine visually with a bright light and a mirror just to be sure nothing has possibly gotten in there. When all is proper, there is literally a high pressure port to high pressure port interconnect that would allow you to blow through back and forth with a blow gun with no resistance. It is an open center hydraulic device and the valve doesn't start closing until you are requesting brake assist by applying pedal pressure (which is how this thing works by applying PS pressure to the backside of the power piston to provide brake amplification).
If you find no foreign objects creating a blockage, and cannot flow through the HP ports freely with compressed air, there
IS a way to coax the spool valve back into a neutral / release position without having to get into the unit. Bolt it back up to the firewall securely, reconnect the pedal rod to the brake pedal. Next, unbolt the master cylinder and coax it out of the way while leaving the brake lines still connected so that you don't have to deal with brake fluid. What you're going to do here is to have a second person get inside and lift the brake pedal upward with about 10-20 pounds of force (don't get tempted to apply more force as it won't help and may hurt). While your helper is lifting upward on the brake pedal, you will be taking a hammer and tapping inward on the master cylinder pushrod carefully - don't blast it with a two pound ball peen, all it's going to take is about the hit required to drive a smaller finishing nail into let's say a wall to hang a 8 x 10 typical picture. With the helper inside lifting up on the pedal + you underhood gently tapping on the MC rod = will dislodge a hung up spool valve. You should then be able to blow back and forth through the high pressure ports without resistance, which means you can now also flow PS fluid. If you go back and look at the exploded view again, you will also see that spool plug at the front of the brake unit that can be removed, but trying to get the little retaining clip out that holds this rascal in is a PITA and this little devil likes to grow wings and fly across the shop into neverland sometimes (we all wear safety glasses, right?). If you do choose to remove the spool plug out of the face of the brake unit, you will be looking for the spool valve inside to be able to slide back and forth in the bore that it operates in.
Give this a good read and then go apply the suggestions to the scenario and let us know how it works out - it *should* take care of business for you. Should you prevail, I suggest you see if you can locate some of this stuff:
http://www.lubegard.com/~/C-198/LUBE...uid+Protectant and also suggest you place a magnet inside of the PS reservoir.
I'll keep my fingers crossed that we can enjoy this end result:
NOTE: I strongly recommend that you do not disassemble the casting halves to attempt service, due the risk of having the accumulator discharge up to 1600PSI of pressurized PS fluid. While there is a very low real time likeliness of actual injury, there was a case many years ago where a dealer service tech died of hydraulic injection into the artery in his wrist while attempting service on a hydroboost unit. This is why Bosch no longer provides individual service parts and requires complete replacement of the entire brake unit if anything what so ever is deemed wrong with this type of brake unit. If you are somebody else reading this and just have to get inside one of these brake units, please firmly depress the brake pedal 10-15 times to discharge any stored pressurized PS fluid in the accumulator cylinder on the side of the brake unit. If the brake unit is already on a work bench, please carefully wrap the brake unit in a thick towel along the casting parting lines / midsection before attempting to separate the case halves to catch any possible high pressure spray of PS fluid.
Brian Hobaugh SCCA National Tour June 2014
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