Ken Brown

Have 1980 R600 changed back from body truck to Prime mover as was original, having trouble getting air through truck tractor protection valve to trailer. valve is very hard to push in and won't stay in. air supply to trailer hoses through valve is low on BOTH supply and service lines when knob held in. (there is low air coming thru BOTH hoses) Also has a air leak thru Park brake control valve when foot service brake applied with park brake applied any info would be appreciated.