Do we have a proportioning valve in (stock) 914s?

I ask, as my brakes aren't really stopping as much as they should. In fact, I tried to lock them up, and couldn't. After stomping the brakes to test, and driving around town, I could smell brake pads. When I got home, I checked to see which were smelling to see if I had a brake dragging or whatever, although I don't think anything is dragging as the car rolls freely. It was my rear brakes. The rims were hot too near the hub, as you would expect after a bunch of hard stops. The fronts were just warm, with no smell. I could touch the hubs easily without burning my finger on the front. What's the deal?

Of note: all my brake parts are new or rebuilt

Replace your brake lines and/or a rebuild is in your future!

I would do the rear brake lines first if you have not done that! They are very difficult to do with the motor in so people tend to put it off!!

Stock prop valve is on the firewall, pretty much right behind the driver's butt. Visible from underneath the car only; it is under the engine shelf.

Double-check the handbrake, make sure the handle is releasing and that the levers on the brake calipers are releasing.

--DD

does`nt sound like your getting much front braking action ...new calipers or rebuilt?? is the bleeder on the front calipers on top??

No. We do not have a proportioning valve in a stock 914. The bias is factory set by the piston and pad sizes.

The device on the firewall behind the drivers seat area is a "Pressure Regulator". It is one of the very first anti-lock brake mechanisms. When it senses a panic stop of 535psi a small micro switch shuts off flow to the rear brakes. As pressure equalizes in the chamber the rear calipers come back into the circuit.

This device helps keep a mid-engine 914 from spinning (high polar moment inertia) in a panic stop.

Jack the rear up, check the runout on the rotors. On the high side of the runout, check the venting clearance. If it is too close you can actually heat up the rotor, pad and caliper causing it to get worse (more heat, expansion, etc.).

I'm guessing old soft lines are buldging internally causing the rears to stay engaged (you can test this while you are checking the runout). After a few minutes or seconds, they slowly release the pressure and the car rolls freely as you describe. Not a fun job. The fronts were probably changed at some point but, because the rears are a PITA they didn't get done.

If you can't lock your fronts, make sure you have great pads and make sure you've bedded them properly.

Thanks folks for the inputs. Just to clarify, I have all new stuff: new 17mm master cyl, new brake pads, new hoses, resurfaced rotors, and rebuilt cailpers. The calipers were done by you, Eric. This stuff was all completed by Mark at Original Customs when I had my car put back into driving cond, so I don't think old worn parts are the issue.

Maybe I just need to bleed? Would the fronts go soft without the back brakes being affected if I need to bleed them? I figured they would all be spongy.

Locked up brakes aren't doing their job effectively. That's why ABS was invented.

The Cap'n

What?

No, Eric. I've seen the guts of the device on the firewall. It is just a spring-loaded valve. You can call it a pressure regulator if you like, because it does limit the pressure going to the rear brakes, or you can call it a proportioning valve which is what most of us do.

It never shuts off flow to the rear brakes that I know of. There is a diagram in the factory manual that shows its effect. Below a certain point (535 PSI sounds like it could be that point) the pressure out matches the pressure in. Past that point, the pressure out still goes up, but not as fast as the pressure in.

There are no microswitches. Nothing electrical about the device at all. It is a spring-loaded valve that reduces the pressure going to the rear brakes if the pressure in the rest of the brake system is high enough. That's it.

--DD

Read the page in the factory manual. It does shut down and a micro switch does not have to be electrical, it just has to be micro.

Understood Cap'n...but mine don't come close.

Did you bed the pads properly?

did you set the rear venting clearance with the e brake cables disconnected from the calipers? does the compensating piece, a t-shaped piece connected to the e brake handle via a small cable, pivot freely? often they're rusted in a fixed position.

k