Geoff Weeks Posted July 16 Share Posted July 16 (edited) The early trailer system HAD NO PARKING BRAKE, they had an emergency/break a way, brake that only held for so long, long enough to chalk the tires to prevent it moving. SOP was to position the trailer, flip the TP control to emergency and chalk the trailer before disconnecting. It is why today, the TP control valve says right on it "Not for parking"! Early TP controls didn't "pop out" when air pressure was lost, often they were simple "flip" valve like are used today for air suspension dump. The TP (TP-1 or TP-2) had the cut-off function built into the actual T-P back behind the cab. With that set-up, if the valve was left in the "normal" position, the trailer would charge and the brakes release with no input from the driver when the pressure built up high enough. It was one of the major changes brought in by -121 brakes, the controls had to pop out to "safe" condition on the loss of air pressure, and remain out until the driver push them back in. edit: Before someone else points it out, I shouldn't have written "chalk" but "chock". A mind is a terrible thing to loose! Edited July 17 by Geoff Weeks 1 Quote Link to comment https://www.bigmacktrucks.com/topic/74335-ltl-spring-brake-upgrade/page/2/#findComment-555320 Share on other sites More sharing options...
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