Swapping proportioning valve for "T" fitting... Options

[davesprinkle]

The proportioning valve is really a simple piston and spring.
I have never met a bad one.

If you still have mushy brakes its because you have not bled the air THROUGH the proportioning valve.
This is not as easy as it sounds.
the spring requires 750 PSI to activate, and most people dont get that when bleeding brakes.
Rich

Rich, respectfully, you're wrong about this. The proportioning valve doesn't OPEN under high pressure, it CLOSES. This behavior has the effect of clamping rear brake pressure while still allowing front brake pressure to rise.

I agree though, that the p-valve traps air.

Want my input? Use the long-hose bleed method. It worked for me and it kept the wife out of the garage. No high-effort stomping on the pedal required.

[70Sixter]

Eric and brant both mentioned tapping on the p-valve with a mallet. That is the old school solution that a lot of new guys don't know about.

And I too got stung by a "pro." After complaining about the spongy pedal the "pro" just popped in a new master cylinder at about 2 1/2 times the going rate plus labor.

I shoulda used my mallet on his head. But I've not been back to that shop in 15 years.

[rhodyguy]

the adj valve is a SUPER idea! (IMG:style_emoticons/default/aktion035.gif) be sure to mount it in the stock location.

frivolity aside. i'm not a brake expert but with the pedle going to the floor, after replacing all of the system components leads me to think (not assume), you may have made an error in your repairs. listen to what eric is telling you. or not...your choise. happy motoring (IMG:style_emoticons/default/driving.gif) . free tees for EVERYONE! FREE THE TEES!! (IMG:style_emoticons/default/shades.gif)

k

[Eric_Shea]

Dave, did you get it working the way you'd like? I'm curious about your impression on the pads. Please share.

Dave is correct on the p-valve. It was always explained to me the other way. I don't think Rich was saying it "opened" though, just activated. Jeff Bowlsby's thread on Factory Manual postings gives the (very interesting) specs as to pressures involved.

If you slam on the brakes and the rears lock up, you will have a hard time controlling the spin... ESPECIALLY in a 914. Mid-engine you say? Correct. I think it's in the first few pages of Vic's handling book. Once a mid-engine car starts a spin it can be much more difficult to control. It basically spins like a top. This is why the 968 with a balanced 50:50 distribution engine in front, tranny in rear, has been the benchmark for handling with the factory engineers.

This is why the p-valve shuts the rear brake off in a panic stop situation.

[SlackeR32]

I don't mean to hijack, but since everyone is already here and I've considered this modification I'd just like to hear some opinon on my situation. My car is not stock, it's a 71 six conversion (3.2) with Carrera calipers/rotors up front, and stock /6 brakes out back. It locks up the fronts way before the rears (I'm not sure if I can lock them up to be honest) and the pedal is not spongy but pretty high effort (not sure if I have 17 or 19 mm MC).

I'd like to increase rear brake bias, so I plan on first trying to adjust the valve. Is it correct that turning the adjuster counter-clockwise increases rear bias? Does anyone know how much can it be adjusted? Will it ever reach the equivalent of a T? Can that screw be backed out completely opening the system, or is there a stop?

Does anyone know a sucessful setup for up a system like mine (with much larger calipers in front) short of a front proportioning valve (I don't like that idea) or dual MC / balance bar setup?

Lastly, since I expect it's my best solution.. Anyone have any experience with balance bars in a street 914? Is there a kit perhaps?

Thanks, and my appologies to the OP.

[roadster fan]

the adj valve is a SUPER idea! (IMG:style_emoticons/default/aktion035.gif) be sure to mount it in the stock location.

frivolity aside. i'm not a brake expert but with the pedle going to the floor, after replacing all of the system components leads me to think (not assume), you may have made an error in your repairs. listen to what eric is telling you. or not...your choise. happy motoring (IMG:style_emoticons/default/driving.gif) . free tees for EVERYONE! FREE THE TEES!! (IMG:style_emoticons/default/shades.gif)

k

You used "frivolity" in a post (IMG:style_emoticons/default/aktion035.gif)

For those not at WCR2008......you shoulda been (IMG:style_emoticons/default/shades.gif)

Jim

[PeeGreen 914]

I have a T fiting in mine. What should I have?

I have 911 fronts and 914 rears with Pagid orange pads. What do you guys recomend I put in? My car is mostly for AX.

[Eric_Shea]

Does anyone know a sucessful setup for up a system like mine (with much larger calipers in front) short of a front proportioning valve (I don't like that idea) or dual MC / balance bar setup?

You have what are called "Wide A-Calipers" up front. This is a standard A-Caliper (very similar to what Jon has as well) with a wider spacer to accommodate the 24mm wide rotor. The standard A-Caliper that Jon has rests on the same 20mm 911 rotor that the M and S-Calipers went on. I reference Jon because he was asking a similar question over the phone a few minutes ago. While I personally think the A-Calipers are heavy, I like the 20mm vented rotor for many reasons.

I'm not a fan of the Carrera system because I think it is too heavy for a 914 as there's generally too much rotating mass (24mm rotors) for such a light car BUT... that being said and the fact that you have the system and 914-6 rear calipers; let's review.

The "bias" is generally set from the factory with piston and pad sizes. This is something that you've fallen off a bit by installing the Carrera calipers up front. The Carrera was the first car to get a larger 42mm piston in the rear, yet it was also the first 911 with a p-valve. The p-valve in a 914 will simply act as a safety feature under extreme braking. By turning the screw counter-clockwise you will take some preload off the regulator spring. This means your rear brakes will be engaged longer in the braking cycle before being shut off (not engaged) -or- you'll have to push harder on the pedal to get it to shut down (stop from braking) the rears. Ideally, you'll want your rear calipers to come loose as soon as the fronts shut down or lock. You never want your rears to lock on a 914. They should ride that ragged edge for best braking.

Because A, S and M-Calipers with their 48mm pistons have been matched up with the rear M-Caliper with it's 38mm pistons on all 911's from 1969 through 1983 I still think you're OK with the brake bias that you have (your rear 914-6 calipers have the 38mm pistons). You just have huge front rotors. Depending on the master cylinder you have, you may want to back out the regulator spring adjuster however... the factory manual shows a lower changeover pressure of 525 psi for a 914-6 (vs. 685 psi for the 914-4). Again, they are comparing 19mm MC vs. 17mm MC. The 914-6 has 48mm pistons up front and 38mm pistons in the rear. The main difference with your set-up is the pad size on the (wide) A-Caliper; it is larger than the 914-6 front M-Caliper. So again... you may want to back the regulator spring adjuster out a bit to compensate.

This is tricky stuff and these are your brakes we're talking about, approach with knowledge and caution. I would have a friend with a video camera set up in an abandoned parking lot. Set up a panic stop staging area and let the video roll. Have a 5mm hex and a 13mm socket available to adjust the valve so the rears are never locked when the fronts are. You will have to break off the JB Weld type sealer the factory has around the adjustment rod to get everything to move (it is very brittle now and usually chips right off). I would back it out one turn at a time until the rears begin to lock. When they lock, screw it in 1/4 turn at a time until they no longer lock up. Once you have it adjusted, JB Weld it back in place and LEAVE IT ALONE.

E.

P.S. Fasteners and spacers from a 911 rear M-Caliper will make your 914-6 rear calipers 914-6/GT calipers. Vented rotors all around. Might be a nice upgrade. Remember... heavier rotating mass again (slight though).

[davesprinkle]

(...lots of insightful braking stuff truncated here...)

I would back it out one turn at a time until the rears begin to lock. When they lock, screw it in 1/4 turn at a time until they no longer lock up. Once you have it adjusted, JB Weld it back in place and LEAVE IT ALONE.

E, I think you've got the adjustment directions backwards. Turning in the adjuster increases diaphragm preload, thus raising maximum rear brake pressure, thus making the rear calipers more likely to lock up.

Summarizing: if you want the calipers to lock up, turn the adjuster in.

Right?

[SlackeR32]

Thank you Eric!
I was a bit confused at first since my name is also Jon, but figured it out eventually. I appreciate you input but I think I probably provided the wrong information. I doubt my rear calipers are off a 6, just the rotors. Does your advice change in that context?

[6freak]

Does anyone know a sucessful setup for up a system like mine (with much larger calipers in front) short of a front proportioning valve (I don't like that idea) or dual MC / balance bar setup?

You have what are called "Wide A-Calipers" up front. This is a standard A-Caliper (very similar to what Jon has as well) with a wider spacer to accommodate the 24mm wide rotor. The standard A-Caliper that Jon has rests on the same 20mm 911 rotor that the M and S-Calipers went on. I reference Jon because he was asking a similar question over the phone a few minutes ago. While I personally think the A-Calipers are heavy, I like the 20mm vented rotor for many reasons.

I'm not a fan of the Carrera system because I think it is too heavy for a 914 as there's generally too much rotating mass (24mm rotors) for such a light car BUT... that being said and the fact that you have the system and 914-6 rear calipers; let's review.

The "bias" is generally set from the factory with piston and pad sizes. This is something that you've fallen off a bit by installing the Carrera calipers up front. The Carrera was the first car to get a larger 42mm piston in the rear, yet it was also the first 911 with a p-valve. The p-valve in a 914 will simply act as a safety feature under extreme braking. By turning the screw counter-clockwise you will take some preload off the regulator spring. This means your rear brakes will be engaged longer in the braking cycle before being shut off (not engaged) -or- you'll have to push harder on the pedal to get it to shut down (stop from braking) the rears. Ideally, you'll want your rear calipers to come loose as soon as the fronts shut down or lock. You never want your rears to lock on a 914. They should ride that ragged edge for best braking.

Because A, S and M-Calipers with their 48mm pistons have been matched up with the rear M-Caliper with it's 38mm pistons on all 911's from 1969 through 1983 I still think you're OK with the brake bias that you have (your rear 914-6 calipers have the 38mm pistons). You just have huge front rotors. Depending on the master cylinder you have, you may want to back out the regulator spring adjuster however... the factory manual shows a lower changeover pressure of 525 psi for a 914-6 (vs. 685 psi for the 914-4). Again, they are comparing 19mm MC vs. 17mm MC. The 914-6 has 48mm pistons up front and 38mm pistons in the rear. The main difference with your set-up is the pad size on the (wide) A-Caliper; it is larger than the 914-6 front M-Caliper. So again... you may want to back the regulator spring adjuster out a bit to compensate.

This is tricky stuff and these are your brakes we're talking about, approach with knowledge and caution. I would have a friend with a video camera set up in an abandoned parking lot. Set up a panic stop staging area and let the video roll. Have a 5mm hex and a 13mm socket available to adjust the valve so the rears are never locked when the fronts are. You will have to break off the JB Weld type sealer the factory has around the adjustment rod to get everything to move (it is very brittle now and usually chips right off). I would back it out one turn at a time until the rears begin to lock. When they lock, screw it in 1/4 turn at a time until they no longer lock up. Once you have it adjusted, JB Weld it back in place and LEAVE IT ALONE.

E.

P.S. Fasteners and spacers from a 911 rear M-Caliper will make your 914-6 rear calipers 914-6/GT calipers. Vented rotors all around. Might be a nice upgrade. Remember... heavier rotating mass again (slight though).

I hope i dont have the same issues when i take all my calipers off ....and when bleeding the system i heard it was good to elevate the back off the car to help with the air in the P-valve??

[Eric_Shea]

E, I think you've got the adjustment directions backwards. Turning in the adjuster increases diaphragm preload, thus raising maximum rear brake pressure, thus making the rear calipers more likely to lock up.

Summarizing: if you want the calipers to lock up, turn the adjuster in.

Right?

The threaded adjust rod simply rests against a hat on the end of the spring. There is a small shut-off valve at the very opposite end of the p-valve on the internal valve assembly. Clockwise would further preload the spring. Counter clockwise should unload... I reserve the right to be wrong. (IMG:style_emoticons/default/confused24.gif)

(IMG:style_emoticons/default/wink.gif)