G. Mann said:
I desperately need a working proportioning valve for a cab chassis F450 with 10 lugs and rear disks.
Haven't been able to find one here anywhere... mine is leaking like crazy.
Just bypass yours with a union fitting till you can find a new one - potentially too much rear braking when empty is a whole lot better than not enough braking while loaded due to leaking high-pressure components.
2stroke said:
I do have thoughts about the prop valve.
1. do I use the F450 one or keep my original one?
If you're asking about the giant aluminum adapter-fitting-like chunk that screws into the rear port of the master cylinder, and then the rear brake lines screws into it, then I'd say reuse your factory one, as it's properly sized for your rear brakes. If you were swapping the rear axle too at the same time, then you should use the F-Superduty valve. As long as they can physically interchange (and in this case they should), those valves need to be matched to the axle, not to the master or booster.
2stroke said:
2. the f450 master has a larger bore than my original is this a problem and again which prop valve to use?
Larger bore is not a problem as long as you use the matching F-Superduty brake pedal assembly. Larger bore with factory brake pedal, not very good idea. Swap pedals, and run the F-Superduty master cylinder. As for prop valve, see above.
2stroke said:
3. I did not check but does the f450 master have only one output feeding the front and rear as my original has 2 outputs one for the front and a seperate one for the rear--I have rear abs I dont know if the f450 had abs and lastly I think not shure does my truck have a low brake fluid sensor--if it does than does the f450 have this as well
Your factory master cylinder has two outputs, and so does the F-Superduty one. There hasn't been a single-output master cylinder since the '60s. Look at how your factory master cylinder is installed, that's the exact same way the F-Superduty one will go in as well - front line screwed directly into master is for front brakes only (it splits in two down at the driver-side brake hose), the rear line screwed into the external valve on the side of the master cylinder is for the rear brakes, both with and without ABS. The F-Superduty may be non-ABS truck, but to you that does not matter, as you're not doing anything with the brake lines. Being an '88 model your factory master cylinder does have the float sensor in the reservoir, the '89 F-Superduty should have it also - it's part of the brakes warning system that is standard in both ABS and non-ABS trucks of those model years. Look at the reservoirs and you'll know, connector for the harness will be in the same general location on both master cylinders.
