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If the brakes are spongy feeling, they may need more bleeding-off of trapped air. Sometimes in custom brake systems, an air trap is built-in and an additional bleed point needs to be added where the air collects .

If the drum brakes have a leading shoe(one sometimes with a longer, bigger friction surface), it should be mounted towards the front of the vehicle. This gives more servo action when the brakes are applied going forward. If the leading shoe is mounted to the back of the vehicle, less servo action is generated(anti-servo?) Mounting the brake backing plates on the wrong side might also cause this to happen.

Also, in a safe manner, try normal stopping with the parking brake slightly applied. If this cures the problem, the brake shoes may need to be adjusted tighter(closer) to the drums.

If these don't fix the problem, try mounting the vacuum pump closer to the booster/vacuum reservoir. You could also mount an auxiliary vacuum reservoir close to the booster.

Maybe the braking actions are not very well balanced and you need a proportioning valve, usually to send more pressure to the front brakes.
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