'72 Suburban - brake proportioning

[WayneE]

Are the brakes on these trucks heavily biased towards the rear? My '72 locks the rears very easily in the wet.

The P/O installed this setup on the truck (sorry for the crappy cell pic):










Front brakes are all new and a quick check of the manual for the proportioning valve/dist block doesn't show any adjustment for front/rear proportioning.

There is adequate vacuum at the booster and the check valve is working properly.

The truck stops well in the dry, but it's a handful in the wet. Just wondering if that's the nature of the beast or if I should be checking for problems.

Thanks!