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Is your matrix the xr model?
Have you double checked that you don't have any vacuum leaks?
It could. Is the vacuum hose that comes from the booster hooked up?
If it is hooked up it may be backwards. There is a valve built into the hose that only works one way.
While its running pull the vacuum hose off the booster and see if there is vacuum present at the hose. If not flip it around.
Any luck with the brakes?