Residual Pressure Valve
- Thread starter POPS
- Start date
Michael Mayben
IHPA Tech Moderator - Retired & No Longer Online
The rpv if present will look something like a rubber disk with a "nipple" segment and a spring, installed in each outlet port of a drum/drum master cylinder. Nothing fancy or hi-tek at all.
Many times the rpv is hidden behind the transition fitting if used on the master cylinder, sometimes they are simply installed in the port before the swivel fitting is screwed in.
A typical rpv for a drum brake channel is a nominal 10psi, many are somewhat less. The same item for a disk brake channel is 2>3psi.
There is no rhyme or reason as to whether or not a new mc will have any rpv present, some do, some don't!
Michael Mayben
IHPA Tech Moderator - Retired & No Longer Online
I realize I didn't completely answer your question!
Since you have the disc/drum combo master cylinder, the rpv for the front calipers is in the rear reservoir outlet connection (or should be!). That would be the 2psi nominal item. However, many times these systems are found to be functional with no rpv for the front bake channel, the "flex" of the caliper piston seal and the dust boot sets the "return" of the caliper piston when the pedal is released. That is what creates the self-adjusting feature regarding the disc brake calipers to allow for pad wear. Disc brake pads will always have a slight drag on the disc when pedal pressure is released, nature of the beast!
It's much more important that an rpv be installed in the drum brake channel which is the front reservoir outlet. That is so the brake shoes don't pull back completely due to the action of the shoe return springs when the pedal is released. We want the brake shoes to be in very close contact with the drum when adjusted properly, but not have to travel from full retraction to full application each time the pedal is pressed. But with a 10psi nominal rpv, there will be a tiny amount of shoe drag on the drum, virtually unnoticeable when rotating the drum by hand.
If you have a cts 2304 service manual, I agree it's extremely difficult to understand in regards to only the Scout II brake system! Most all of the brake section is lifted from the master manual, cts 2300 which contains brake information for virtually every ihc-produced light and medium duty vehicles. It covers every master cylinder, booster system, and brake set ever used. But you must know exactly which brake component you are dealing with in order to home in on the right sections, these manuals are real difficult to deal with for a hobbyist, they were intended only for use by trained ihc service mechanics and dealer personnel.
That said, look on page 9 of the cts2470q section in your manual,...figure 16,...and you will see a description of a typical rpv. That is for an oem master cylinder though. There were many variations of the master cylinders produced by the aftermarket, that is what we deal with today, and therefore, there are many variations of the rpv situation after 35 + years! Thus, there is no single correct explanation to your question!
Furthermore, this kind of brake system variation is not peculiar to only ihc stuff, it's generic for all domestic vehicles of that era! Disc brakes were still a fairly new "innovation" for the domestic car and truck industry at that point in time and in many cases the disc systems were very marginal in performance.
Welcome to another texian...I've spent many hours/days in lubbock, lived in abilene during the formative years for gearheads! Home is around fort worth!
Since you have the disc/drum combo master cylinder, the rpv for the front calipers is in the rear reservoir outlet connection (or should be!). That would be the 2psi nominal item. However, many times these systems are found to be functional with no rpv for the front bake channel, the "flex" of the caliper piston seal and the dust boot sets the "return" of the caliper piston when the pedal is released. That is what creates the self-adjusting feature regarding the disc brake calipers to allow for pad wear. Disc brake pads will always have a slight drag on the disc when pedal pressure is released, nature of the beast!
It's much more important that an rpv be installed in the drum brake channel which is the front reservoir outlet. That is so the brake shoes don't pull back completely due to the action of the shoe return springs when the pedal is released. We want the brake shoes to be in very close contact with the drum when adjusted properly, but not have to travel from full retraction to full application each time the pedal is pressed. But with a 10psi nominal rpv, there will be a tiny amount of shoe drag on the drum, virtually unnoticeable when rotating the drum by hand.
If you have a cts 2304 service manual, I agree it's extremely difficult to understand in regards to only the Scout II brake system! Most all of the brake section is lifted from the master manual, cts 2300 which contains brake information for virtually every ihc-produced light and medium duty vehicles. It covers every master cylinder, booster system, and brake set ever used. But you must know exactly which brake component you are dealing with in order to home in on the right sections, these manuals are real difficult to deal with for a hobbyist, they were intended only for use by trained ihc service mechanics and dealer personnel.
That said, look on page 9 of the cts2470q section in your manual,...figure 16,...and you will see a description of a typical rpv. That is for an oem master cylinder though. There were many variations of the master cylinders produced by the aftermarket, that is what we deal with today, and therefore, there are many variations of the rpv situation after 35 + years! Thus, there is no single correct explanation to your question!
Furthermore, this kind of brake system variation is not peculiar to only ihc stuff, it's generic for all domestic vehicles of that era! Disc brakes were still a fairly new "innovation" for the domestic car and truck industry at that point in time and in many cases the disc systems were very marginal in performance.
Welcome to another texian...I've spent many hours/days in lubbock, lived in abilene during the formative years for gearheads! Home is around fort worth!
Last edited:
Puddle Jumper
Member
Hey all.
Does anyone know the correct part# for a new replacement m/c for a 79 sii 345/auto 4x4?
Thanks ahead.
Does anyone know the correct part# for a new replacement m/c for a 79 sii 345/auto 4x4?
Thanks ahead.
Michael Mayben
IHPA Tech Moderator - Retired & No Longer Online
Hey all,
I bought a "new" stock mc locally for my truck that has a rear disk brake conversion. The conversion is the same brand but not sourced from Jeff. Had to call Jeff to buy two "sleeves" for the cable mounting bracket on the calipers and weld 'em in myself.
Anyways, after driving for maybe 5 miles the truck would slow down and wouldn't move, like I had the brakes on...rpv! I would sit there for 15 minutes until they cooled down, drive another 5 miles, stop, cool down and made my way home that way. Missed easter festivities w/the family.
Once removed, I stuck a paper clip into the rear port on the mc and there was resistance from the valve. There was no such resistance on the front. I bought another "new" mc and no such valve in the rear. Brakes are great now.
The moral of my story: buy everything from Jeff.
I bought a "new" stock mc locally for my truck that has a rear disk brake conversion. The conversion is the same brand but not sourced from Jeff. Had to call Jeff to buy two "sleeves" for the cable mounting bracket on the calipers and weld 'em in myself.
Anyways, after driving for maybe 5 miles the truck would slow down and wouldn't move, like I had the brakes on...rpv! I would sit there for 15 minutes until they cooled down, drive another 5 miles, stop, cool down and made my way home that way. Missed easter festivities w/the family.
Once removed, I stuck a paper clip into the rear port on the mc and there was resistance from the valve. There was no such resistance on the front. I bought another "new" mc and no such valve in the rear. Brakes are great now.
The moral of my story: buy everything from Jeff.