Vehicles equipped with load-sensing brake proportioning (LSP) valves must be serviced properly or brake performance may suffer. Obviously, ride height changes according to the load the vehicle carries. The LSP valve senses vehicle height or load via a link connected to the suspension.
These valves, which are popular on multi-purpose vehicles such as station wagons, minivans and sport-utility vehicles, enhance handling by adjusting rear brake pressure according to vehicle load.
Generally speaking, the greater the load, the lower the ride height. The lower the ride height, the more braking pressure the valve routes to the rear brakes.
Technicians at Gabriel Ride Control Products caution that a change in ride height may make a noticeable difference in front-to-rear brake balance. For example, if fatigued springs allow the rear of the vehicle to sag, the rear wheels may lock up during light-to-moderate braking. When the vehicle is equipped with ABS, this condition may cause a complaint of overactive antilock operation.
Likewise, customers who customize the vehicle by raising or lowering it also disturb the prescribed ride height/proportioning valve relationship. Of course, jacking up the vehicle reduces pressure to the rear brakes.
The safest, surest way to maintain the original brake balance is to measure ride height to confirm that it's within manufacturer's specifications. Then, where necessary, adjust the proportioning valve-to-suspension link or rod according to the manufacturer's recommended procedure.
Checking and correcting ride height is a prerequisite for a successful wheel alignment. Many technicians have found that checking the link or rod adjustment is an important post-alignment procedure. Eric Lang, a technical specialist at EIS Brake Parts, said routine rear toe adjustment on a Ford Taurus or Mercury Sable can make a noticeable difference in brake balance.
Timothy Steigerwalt, service manager at Kieser's Tire & Service Center, Fairless Hills, Pa., said his technicians have encountered the condition Mr. Lang described. As a result, they check the proportioning valve link adjustment after performing a rear-wheel alignment on Taurus/Sable suspensions.
Finally, Mr. Lang reminded technicians to bleed the brakes with the vehicle sitting static-preferably on a drive-on lift. If the rear wheels are allowed to hang freely, the LSP valve may prevent bleeding the rear brakes properly, he said.