My 4X4 has drum brakes with vaccuum boost on all 4's. About 1/2 the time when depressing the brakes they go down further than they should and re-depressing them brings them up. I have gone thru the whole system, new Master (twice)/ Wheel cylinders, shoes are adjusted and bled the lines several times. This has been going on for a long time and has gradually gotten to its present stage. Not dangerous but very annoying. Could a malfunction in the "Vaccuum boost" assembly cause this??

 

any major rpm changes? might be booster if yes

 

It idles good, vaccuum reading is good, the truck is very strong with about 400 cubes.