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Where to start, I'm having this brake problem thats been a major issue now with my 89 comanche upon upgrading to a willwood prop valve and 96 booster/master


Run down- new NOS chrysler 95-96 booster and master

-Willwood prop valve

-96 pedal assy and brake switch

-new hard line goin back to rear (eliminated height sensing valve)

-calipers where rebuilt with new seal and dust seal also new boots and slide pins

-rear drums were adjusted to just lightly drag

-new extended braided lines


The Problem- The brake pedal builds pressure after several pumps then loses it by the time I return the pedal for a second pump,I hear a slight hissing noise then a couple light popping noise from around master cylinder.

There are no fluid leaks. I have had (2) orielly's masters on here and a oreilly's booster that did the same thing,this is why i went with a oem NOS unit.I believe the problem lies elsewhere then the master because of this reason.

Master cylinder was bench bled and brake system was bled with a motive power bleeder.All flares where made with a eastwood flaring tool so they are very good,double flares everywhere besides the two at the master are bubble flared.

When the vehicle is running the pedal gets even softer and builds no pressure (booster is working properly) but fluid lowers to the "add" when pumping brakes then returns to the "full" slowly if brake pumping is stopped.

I have weeks into trying to figure this out,give me your best shot with this basket case!

Thanks in advance!

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Well, i'll be honest i don't know for a 99 XJ, but from the description you gave the calipers would be on upside down if they were MJ calipers. On my Comanche the bleeder valve is just above the hydrualic hose.


They can be put on upside down simply by putting the one meant for the Driver side on the Passenger and vise versa. They will never ever bleed properly. But, I wouldnt bother swapping them until someone else chimes in.

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If the bleeder screw is on top, the calipers are correct.


Gotta ask: What did you use for the distribution block? Since you eliminated the MJ load sensing proporioning valve and assuming it is plumbed correctly, did you retain the original MJ distribution block, or swap in an XJ block with the proportioning valve?

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  • 2 months later...

I doing the same thing as you but just upgrading my master and booster from a new OEM 96 XJ. I am actully doing this conversion hopefully this weekend and I will post pictures of my process for future reference. Probably a dumb question but I have never done this before.... Do I have to bleed my new master cylinder before installation?

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It's always a good practice to bench bleed a master cylinder before installing, especially if it's a new dry unit. However when I did mine I used a "previously owned" master with no bench bleeding and had no problems.

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