Residual pressure valve

[diesel_lv]

I installed front discs, rebuilt rear drums w wheel cylinders, new aluminum 15/16" master cylinder on '67 Coronet. Work great. Except slowly get squishy and get to no brake at all after about 2 weeks. No fluid anyplace. Tried all suggestions, loosen then tighten all connections, bleed again etc. Emailed Cass at Dr Diff. He recommends 10 lb Pressure Relief Valve for rear brakes. Has anyone here ever had to do this and if so, did it work. Not real keen on cutting brake line, flairing both cut ends and adding valve. Thanks

 

[Nate S]

That's bizarre. My old '55 dodge pickup had a vented master cyl and needed to be refilled once a year due to evaporation. Did a similar upgrade on my '64 fury and have had no similar issues, nor need for any valve other than proportioning.

 

[diesel_lv]

That's bizarre. My old '55 dodge pickup had a vented master cyl and needed to be refilled once a year due to evaporation. Did a similar upgrade on my '64 fury and have had no similar issues, nor need for any valve other than proportioning.

That's what I would have thought. Here is his quoted response "Are you using modern rear wheel cylinders? If not, the system will suck air through the rear wheel cylinder cups". I am using the wheel cylinders he sells in the drum brake rebuild kit. Hoping someone else has had to do it. Otherwise I would think he'd have a * someplace saying "will need modern wheel cylinders" or "will need residual pressure valve".

 

[khryslerkid]

What's a "modern" wheel cylinder?

After you re-bled the system how did they feel?

A master cylinder can have a internal leak. They can leak past the piston internally or out the back. Out the back you need to check the boot for fluid.

On a internal leak, pump up the pedal and then just apply pressure lightly and see if it starts to drift down. Add a little more pressure then a light amount and try to get it to drift down again.

 

[diesel_lv]

What's a "modern" wheel cylinder?

After you re-bled the system how did they feel?

A master cylinder can have a internal leak. They can leak past the piston internally or out the back. Out the back you need to check the boot for fluid.

On a internal leak, pump up the pedal and then just apply pressure lightly and see if it starts to drift down. Add a little more pressure then a light amount and try to get it to drift down again.

The brakes feel great after bleeding every time. As the get worse over a 2 week time frame, they go to no brakes. Pumping them does not help. Am not losing brake fluid anyplace, firewall, distribution block, rear T, calipers, rear wheel cylinders etc... when 1st bleed, pedal stays firm, even on brake at very long red light.

 

[khryslerkid]

So bleeding the lines will produce air?

Try playing with the pressure on the pedal, try to get it to drift down. Sometimes just a very light amount of pressure can get the piston to bypass the fluid internally.

What brake fluid are you using, Dot 3 ?

 

[Nate S]

It’s going somewhere. Inside the brake booster? Only place I can think that would hold that volume.

 

[69L48Z27]

Residual valves will keep up the slack on drums that are under adjusted. Or, they’re used if the MC is below the wheel cylinder or caliper.

67 Dodge has ‘modern WC’s’.

 

[Beekeeper]

When class says modern wheel cylinder he means the new style that has the residual valve in them. The early ones didn’t and the valve was in the master cylinder outlet.

 

[diesel_lv]

When class says modern wheel cylinder he means the new style that has the residual valve in them. The early ones didn’t and the valve was in the master cylinder outlet.

Correct, and the "new" aluminum master cylinders don't have it built into them. He said the 73-74 A bodies power brakes had a cast master cylinder. But not sure if the have a residual pressure valve. I guess I'll be cutting and flairing the brake line

 

[Nate S]

Learn something every day! I guess they just keep enough pressure in there to keep the rubber cups in the cylinder expanded and not leaking. That would mean all that fluid went into your rear drums. Might want to have a look in there. Seems like a pretty significant oversight on the part of the kit supplier if you told them what you were doing.

 

[diesel_lv]

Learn something every day! I guess they just keep enough pressure in there to keep the rubber cups in the cylinder expanded and not leaking. That would mean all that fluid went into your rear drums. Might want to have a look in there. Seems like a pretty significant oversight on the part of the kit supplier if you told them what you were doing.

No, no fluid leaks. My original post clearly states that. Just getting air in the system.

 

[Beekeeper]

The residual valves in the wheel cylinders hold a small amount of pressure in the cylinder and
Hold the shoes out close to the drum. Without a residual valve in the wheel or master cylinder
There will be a ton of travel in the pedal before the shoes make contact with the drum. Sounds
like exactly what your having an issue with once you bleed they are fine but when they are not used for a period of time the shoes force the wheel cylinders back in.

 

[diesel_lv]

The residual valves in the wheel cylinders hold a small amount of pressure in the cylinder and
Hold the shoes out close to the drum. Without a residual valve in the wheel or master cylinder
There will be a ton of travel in the pedal before the shoes make contact with the drum. Sounds
like exactly what your having an issue with once you bleed they are fine but when they are not used for a period of time the shoes force the wheel cylinders back in.

That all makes sense. But im curious why my front brakes wouldn't work. They are properly adjusted to about a 70/30 front to back.

 

[Beekeeper]

One other thing to mention here is that there was a run of bad master cylinders may need to try a different one. In this day and age new doesn’t mean good.

 

[Nate S]

The residual valves in the wheel cylinders hold a small amount of pressure in the cylinder and
Hold the shoes out close to the drum. Without a residual valve in the wheel or master cylinder
There will be a ton of travel in the pedal before the shoes make contact with the drum. Sounds
like exactly what your having an issue with once you bleed they are fine but when they are not used for a period of time the shoes force the wheel cylinders back in.

I don't believe this is correct. That's what brake adjusters are for. A good example is when brakes are loosely adjusted the pedal has a long throw, when tightly adjusted they are right at the top. If this valve kept the cylinders out they would always be at the top. According to a couple sources (strange has a nice writeup) the pressure is around 10 psi, this wouldn't hold back the springs for sure.

 

[Beekeeper]

I don't believe this is correct. That's what brake adjusters are for. A good example is when brakes are loosely adjusted the pedal has a long throw, when tightly adjusted they are right at the top. If this valve kept the cylinders out they would always be at the top. According to a couple sources (strange has a nice writeup) the pressure is around 10 psi, this wouldn't hold back the springs for sure.

This is pretty good info on it
https://techtalk.mpbrakes.com/how-to-series/brake-valve-overview