Downhill braking while towing, some thoughts on LSPV and rear brakes

Something to ponder over about downhill braking
There has been a bit of discussion recently about towing and braking problems so I thought I would throw this up for discussion from the experts.

Something to think about for down hill braking is the LSPV (Load sensing proportioning valve)
On previous vehicles this was a manual valve affected by load in vehicle which also changed as you go down a hill putting more braking to the front of the vehicle.
On the 100 with power assisted booster, the LSPV is built in to the master cylinder and gives the following braking pressures

FRONT 2,256 kPa (23 kgf·cm2, 327 psi) REAR 2,452 kPa (25 kgf·cm2, 356 psi)
FRONT 4,413 kPa (45 kgf·cm2, 640 psi) REAR 3,334 - 3,727 kPa (34 - 38 kgf·cm2, 484 - 540 psi)
FRONT 7,845 kPa (80 kgf·cm2, 1,138 psi) REAR 5,197 kPa (53 kgf·cm2, 754 psi)

If you look at the first line, you find at these pressures the rear is getting more pressure than the front (yes I know the piston/pad/rotor surface areas are different) but it shows that under the possible lighter continual braking going down hills, that the rears are going to be doing a fair bit of work.

This possibly causes heat and overwork of the rear calipers in this driving situation.
As you can see the more pressure applied to the brakes, the greater change in percentage braking from front to rear. So when you moderately hang the brakes on or slam them on hard, you get lots of front braking, but when applied lightly the rears have a work out! Something to keep in the back of the mind for downhill driving I believe. My .02, any comments, corrections or more technical info appreciated.

Excellent info. That may tell me why the rears seem so engaged downhill!

That may also help explain why the rear pads wear out quicker than I had expected - especially for those of us who try to be "easy" on the brakes. Very interesting information.

100 TD, simply grand the stuff that you post.




When I apply a good amount of force, the front slotted rotors and high performance pads bite down hard and haul my truck to a quick stop. When I apply the brakes lightly, they don't seem to bite much at all and the rears seem to do more. Certainly empirical data, but worth noting based upon your numbers.

Perhaps a more moderate application of the brakes is appropriate versus a light touch?

Although harder braking will utilise the front to rear brake ratio more effictively and comparatively reduce rear use, I don't think harder braking, as compared to lighter braking is the answer. Harder braking slows the vehicle speed and then you allow increase in speed then slow again, a bit of a see saw effect, where controlled smooth braking would be more ideal, albeit without the problem of overheating brake components. Due to the systems inherrent design, I believe the utilisation of enging braking to the maximum is the best option to avoid prolonged braking and the overheating of components and the possiblity of boiling the fluid and losing brake control.
(As mentioned I don't have the piston diameters/pad surface area/rotor diameter info. If someone got the info and did some calculations you could work out actual braking forces applied at varying pressures and compare front to back realistically. This would be nice to know, but due to the fact you cannot adjust the LSPV, it would not do a lot for us. We have to live with what is there, but being aware of it is worthwhile I believe.) My .02

Agreed. I just monitor my rear pads more than any other vehicle I have owned. Going back to my bicycle days, I liked using the rear caliper brakes as much as possible, with a lighter touch on the front, for smoother stops.