Rear brake pressure regulator valve, Brake repair Options

[DamonsCarrera]

The brake pedal on my 914-6 is very spongy, even after repeated bleeding with a pressure bleeder. I isolated the problem to the brake-proportioning valve or pressure regulator. I cannot seem to get the brakes properly bled with this valve in the system.

I've read other posts by some here and it seems this valve is a source of some debate . Is there some trick to bleeding the brakes with it? Do I need to bleed the brakes the old-fashioned way, have someone push on the brake pedal?

[Eric_Shea]

Wrong move. All the Tilton can do is reduce fluid flow to your rear calipers effectively reducing your overall braking performance. (IMG:style_emoticons/default/thumb3d.gif)

[ChrisFoley]

Wrong move. All the Tilton can do is reduce fluid flow to your rear calipers effectively reducing your overall braking performance. (IMG:style_emoticons/default/thumb3d.gif)

That's very inaccurate.
Braking systems don't' rely on flow, they rely on pressure. Once the pads are in contact with the rotors there is almost no fluid flow.
A good proportioning valve allows normal braking pressure up to the adjustment level, then reduces the rate at which the pressure increases.
If done properly it is every bit as effective as the factory regulator, maybe better.
The issue is that adjusting to the correct setting by doing brake tests on the road may be dangerous.

In fact, a poorly adjusted stock regulator will reduce overall braking performance even more than a modern proportining valve, because it takes a considerable amount of brake fluid to move the piston inside the regulator, allowing the pedal to continue moving with little increase in braking force.
In my experience, a large proportion of the stock regulators out there need substantial adjustment at this point to be effective.

[Jeff Hail]

Wrong move. All the Tilton can do is reduce fluid flow to your rear calipers effectively reducing your overall braking performance. (IMG:style_emoticons/default/thumb3d.gif)

That's very inaccurate.
Braking systems don't' rely on flow, they rely on pressure. Once the pads are in contact with the rotors there is almost no fluid flow.
A good proportioning valve allows normal braking pressure up to the adjustment level, then reduces the rate at which the pressure increases.
If done properly it is every bit as effective as the factory regulator, maybe better.
The issue is that adjusting to the correct setting by doing brake tests on the road may be dangerous.

In fact, a poorly adjusted stock regulator will reduce overall braking performance even more than a modern proportining valve, because it takes a considerable amount of brake fluid to move the piston inside the regulator, allowing the pedal to continue moving with little increase in braking force.
In my experience, a large proportion of the stock regulators out there need substantial adjustment at this point to be effective.

I'm going to jump in Chris's passenger seat and ride shotgun on this one.

Flow and pressure are two different aspects of the system. Everything upstream of the regulator or proportioning valve relies on normal pressure dictated by the design. When the adjustment level is dialed in say down from the maximum a reduction in working pressure occurs. Even if upstream pressure is increased down stream pressure is regulated proportionally by its limiting orifice .

In the oem regulator downstream pressure is regulated by the same and piston movement in its bore resisted by spring tension. Here the upstream flow is changed in pedal travel where as the downstream pressure and flow is still limited by the lines and caliper pistons/ bore.

Lets throw a cog in for good measure. The oem master cylinder utilizes a dual circuit front and rear inside a common bore. If you disassemble a master cylinder you will quickly realize one circuit compresses against the other in its travel. What most people don't understand is the dual circuit masters actually create a bias within themselves. One circuit compresses less. Simply a conventional dual circuit inline master cylinder will not give you 100% equal pressure or volume out of both circuits throughout its travel.. That's why we have double master cylinder pedal options with bias adjustment. Yeah -I know its a tangent but so are pressure curves in closed system fluid dynamics.. (IMG:style_emoticons/default/stirthepot.gif)


Love you both. Happy holidays Gentlemen!