Took the hydroboost apart and replaced the seals with a rebuild kit to get rid of a leak. Put everything back together and reinstalled it in the car. When turning on the engine, and bleeding the air out the system, the break pedal would self apply almost completely. Applying the gas would make the brake pedal go down further. Turning the wheel lets the brake go up a little.

There appears to be an excess of pressure. Reading about it, people often say the dump valve was likely not seated correctly. Not sure where fluid goes when it goes past the dump valve, I took the hydroboost apart again to flip the dump valve over and reinstalled it only to find the same problems.

Is there anything else that could be causing this issue? I noticed when taking it apart the first time, there were some grooves on the spool valve where o-rings would be, but I don't remember seeing any the first time taking it apart, so I left it without o-rings. Could that be causing the issue?

Here is the diagram on the instructions that came with the rebuild kit:

enter image description here

  • Reassembling with the spool assembly inserted backwards or without a required O ring could cause the symptoms you are describing. A series of photos as you disassemble would help next time you tackle something like this.
    – MTA
    Jun 5, 2023 at 15:57

1 Answer 1


So it looks like the issue was that I did not attach the bracket arm (I don't know what you call it) that is on the input rod & piston assembly to the end of the head of the spool assembly. See the picture below:

enter image description here

These two need to be attached. There are no o-rings or seals that go on the spool assembly. After rebuilding the hydroboost again and making sure I done this step, the brakes worked properly again.

The source of the video where I made this screenshot is: https://youtu.be/1tr3vHl1zhA?si=8WNnEAwhvsXQaLH2

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