Adjustable Proportioning Valve

[Golly]

My understanding is : drum brakes have return springs between the shoes, holding pressure against the wheel cylinders. Unless there is a residual line pressure valve in the system (10 lb) they will push fluid back towards the master cylinder, no matter where it is. Discs need only 2 lbs to keep the pads touching the rotors, otherwise they slowly back off too. Some master cylinders have valves in the master cylinder, ( a little rubber duck bill looking thing where the brake lines screw in) , some are in the factory proportioning valve that splits lines front to rear etc on a dual master cylinder system. and yes there are lots of people that don't use them and are doing OK , but every car I've built has had, in line residuals in it.

 

[TriodeLuvr]

Theoretically in a full system fluid won't drain back from the brakes into the master cylinder regardless of the relative heights of the two; there is no way for air to get into the brakes to allow the fluid to drain.

Air doesn't have to enter the system for this to happen. If the piston in the slave retracts, fluid will move into the master. It has no place else to go. The thing is, I've never replaced disc pads on any system - car or bike - where it didn't require a C-clamp to force the piston back into the caliper. I'm having trouble with the idea that so much force would be required if in fact there's no valve in the master cylinder resisting this backflow.

Jack

 

[TriodeLuvr]

My understanding is : drum brakes have return springs between the shoes, holding pressure against the wheel cylinders.

Yes, and after reading more on this subject, I see now that some MCs have residual valves, others don't. Good discussion, I never knew that and wouldn't have looked it up otherwise. I guess this means that if a vehicle gradually loses pedal firmness, but the system is free of air, it might be the RPV. This raises the question of how one would know if only one RPV has failed in a dual system. Poor quality (unbalanced) braking, I guess. That could be a bear to troubleshoot without a line pressure gauge.

Jack

 

[PotvinGuy]

The return springs in drum brakes serve to retract the shoes to a resting position. These springs do not then continue to force fluid out of the brake cylinder. And the brake cylinder has an internal spring that maintains the piston positions and the fluid in the cylinder. Here's a nice animation:

Disc pistons don't have any mechanism to cause them to retract into their bores; in fact the piston seal acts to prevent this. Here's an article with a paragraph on how disc brakes adjust and why a retraction tool is needed: http://en.wikipedia.org/wiki/Disc_brake
When replacing pads, another reason force is needed to press disc brake pistons back into the bores is because the pistons have a large area and the brake lines are small and long, and the fluid is viscous. And if there are residual pressure valves in the lines or master cylinder even more force will be needed.

 

[TriodeLuvr]

Just to follow up, the new Speedway purple proportioning valve came in today, and it was a bolt-in replacement for the older style. Tried to take the old one apart after the job was done, but couldn't figure out how to remove the shaft from the retaining nut assembly. I suspect the shaft is threaded through the retainer from the inside (there's a screwdriver slot cut into the end), then the knob is pressed or epoxied onto the shaft. Anyway, I'll bleed the rear brakes tomorrow, and hopefully that will be the end of this for awhile.

Jack

 

[choppedtop]

Good video. I understand about the fluid not suppose to leak back down to the master. Mine did. Not leaks from day one. After over 3k miles still no leaks and no bleed down. Oh well, my thought is use the residual valves if you think you need them, can't hurt.
Lee