Last week my wifes Gen 3 hydraulic brake booster finally gave up while she was on her way from my brother in laws house. The ABS light and Brake light came on and then a constant buzzing sound from the dash suddenly..so she pulled into a neighborhood. She still had brakes so she called me and told me the issue and she was about less than a mile from home so I said try to make it home. When she proceeded to go home she lost her brakes completely...pedal got very stiff with little to no brake pressure, she pulled into a super market parking lot relying on her E-brake.
I got home and started to diagnose and after some quick research I read the culprit is the pump motor so I checked for that 1st. I turned the key on the "ON" position and you should hear the pump/accumulator cycle but I didn't anything at all. After attempting to check a couple times the pump was definitely the pump motor.
I ordered a HBB unit from a fellow Montero owner from the FB for sale groups and he came through with a very reasonable price and shipped it out promptly. Once I received it i prepped the unit and proceeded to install it and go through the normal way I always bleed my systems for year with my bleeder kit. I was having no luck...pedal was going straight to the floor even with pressing the pedal 30-40x's to prime the accumulator. I was hearing and seeing the pump cycling when I had the key on the ON position but I couldn't get the system to bleed correctly.
I found this article on a Pajero owners group and followed the steps they posted and it worked for me. Also this link has tips on diagnosing and repairing the HBB pump motor. I found out the Toyota and Lexus share the similar Aisin HBB units. The design of the units are different..but seems they all use the same pumps at the Gen 3 Monteros so this allows for more options to find a replacement motor.
https://www2.pajeroclub.com.au/forum/showthread.php?t=46516
Monty by
grimgs, on Flickr
The original failed unit
Monty by
grimgs, on Flickr
The new "used" HBB unit
Monty by
grimgs, on Flickr
I will most likely get the original failed HBB sent out for a rebuild. Rockauto has a procedure to send your HBB out for a rebuild...I believe its $300 to get it rebuild.
Monty by
grimgs, on Flickr
Taking out the HBB is really simple.
-x4 brake lines on the unite (x2 on the front and x2 on the side where the accumulator sits
-Disconnect the pin where the brake pedal attaches to the HBB unit which is a cotter pin and the lock pin slides out.
-x4 12mm mounting nuts inside firewall.
I was eventually successful bleeding the system and after driving the Montero for 30 mins...the HBB unit sorted itself out and pedal feel got closer to normal and the ABS and BRAKE light eventually went off including the buzzing sound after the HBB unit cycled a few times.
