Master cylinder question
Cargo Master
Joined: Mar 2002
Posts: 3,132
Likes: 218
From: W (BY GOD) V
Are you asking about the RESIDUAL PRESSURE VALVE(S)?
On DRUM BRAKES, a 10# RPV (or wheel cyl expansion cups) is required.
The purpose is to not allow the pistons to return so quickly (brake shoe spring tension) on pedal release as to allow the piston cups to partially collapse and allow air to be ingested into the system or have fluid leakage.
This and proper shoe adjustment is what dictates correct pedal height on a DISC/DRUM application.
The PRV only maintains enough pressure to prevent cup shudder. It does not hold the shoes out.
On DRUM BRAKES, a 10# RPV (or wheel cyl expansion cups) is required.
The purpose is to not allow the pistons to return so quickly (brake shoe spring tension) on pedal release as to allow the piston cups to partially collapse and allow air to be ingested into the system or have fluid leakage.
This and proper shoe adjustment is what dictates correct pedal height on a DISC/DRUM application.
The PRV only maintains enough pressure to prevent cup shudder. It does not hold the shoes out.
Mountain Pass
Joined: Nov 2012
Posts: 109
Likes: 10
From: Hurst,Texas
Are you asking about the RESIDUAL PRESSURE VALVE(S)?
On DRUM BRAKES, a 10# RPV (or wheel cyl expansion cups) is required.
The purpose is to not allow the pistons to return so quickly (brake shoe spring tension) on pedal release as to allow the piston cups to partially collapse and allow air to be ingested into the system or have fluid leakage.
This and proper shoe adjustment is what dictates correct pedal height on a DISC/DRUM application.
The PRV only maintains enough pressure to prevent cup shudder. It does not hold the shoes out.
On DRUM BRAKES, a 10# RPV (or wheel cyl expansion cups) is required.
The purpose is to not allow the pistons to return so quickly (brake shoe spring tension) on pedal release as to allow the piston cups to partially collapse and allow air to be ingested into the system or have fluid leakage.
This and proper shoe adjustment is what dictates correct pedal height on a DISC/DRUM application.
The PRV only maintains enough pressure to prevent cup shudder. It does not hold the shoes out.
after bleeding the brakes yesterday back it out of the shop to clean the shop floor of all oil, transmission and brake fluids and snapped some pictures.
Going to Summit for new rims Saturday morning.
Last edited by DCrusher; Apr 2, 2026 at 10:54 AM.
Cargo Master
Joined: Mar 2002
Posts: 3,132
Likes: 218
From: W (BY GOD) V
Is the MC a recent service replacement? The vendor should be able to tell you if it has an RPV in the drum circuit. Save you a lot of trouble.
Recent SERVICE REPLACEMENTS (CHI-COM) usually do not include the valve(s). It is a crap-shoot.
Your front calipers, are they OEM or a conversion using GM DELCO low-drag calipers?
If no internal valve, you can add an external inline valve or put a cup expander style kit in the wheel cylinders.
You have the rear brakes adjusted correctly and they have worn in (new shoes)? It may take some miles before they seat properly and then after proper adjustment the self-adjuster feature will take over (if it is functional).
Recent SERVICE REPLACEMENTS (CHI-COM) usually do not include the valve(s). It is a crap-shoot.
Your front calipers, are they OEM or a conversion using GM DELCO low-drag calipers?
If no internal valve, you can add an external inline valve or put a cup expander style kit in the wheel cylinders.
You have the rear brakes adjusted correctly and they have worn in (new shoes)? It may take some miles before they seat properly and then after proper adjustment the self-adjuster feature will take over (if it is functional).
Last edited by KULTULZ; Apr 2, 2026 at 11:21 AM.
Cargo Master
Joined: Mar 2002
Posts: 3,132
Likes: 218
From: W (BY GOD) V
Okay folks I have to step in ask a question I have done a Disc/Drum conversion and my MC is a Disc/Drum MC. should I have these on both or just the rear drum brakes?? The rear drum are the original brakes nothing has been changed. I just bleed the brakes after setting for almost 20 years in the shop and I’m still not really happy with the brakes they’re kinda spongy and to me the pedal is going way further than I want it to plus I added a power brake booster when it was a NON power brake truck.
My reading comprehension seems to be lacking (OLD FART ADHD).
The truck sat for twenty years? Are you sure the rear cylinders are not sticking/frozen?
The booster add-on. Did the kit maker mention anything about booster pin the MC piston correct adjustment?
What style front disc did you use?
Last edited by KULTULZ; Apr 2, 2026 at 11:31 AM.
Mountain Pass
Joined: Nov 2012
Posts: 109
Likes: 10
From: Hurst,Texas
My reading comprehension seems to be lacking (OLD FART ADHD).
The truck sat for twenty years? Are you sure the rear cylinders are not sticking/frozen?
The booster add-on. Did the kit maker mention anything about booster pin the MC piston correct adjustment?
What style front disc did you use?
everything came from donor trucks around the 72 time line
which the only thing I can’t tell you is if I bought a 72 and above MC or not. I’m thinking I did get a 72 MC
Last edited by DCrusher; Apr 2, 2026 at 12:31 PM.
Cargo Master
Joined: Mar 2002
Posts: 3,132
Likes: 218
From: W (BY GOD) V
which the only thing I can’t tell you is if I bought a 72 and above MC or not. I’m thinking I did get a 72 MC ...
Well, even then you don't know unless you check for it being there. It all depends on the quality of part by the vendor.
You ask a seller and they go mentally off-line, source elsewhere.
Make sure the wheel cylinders are not frozen.
'My 'LITTLE THUMBSCREW' likes to use her foot as a disciplinary tool ...
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
2027 Ford Super Duty Buyer's Guide (Every Model, Engine, & Package)
Brett Foote
Top 10 Ford Truck Tragedies
Joe Kucinski
AEV FXL Super Duty - the Super Duty Raptor Ford Doesn't Make
Brett Foote
Lobo Vs Lobo: Proof the F-150 Lobo Should Be Even Lower!
Michael S. Palmer
Ford's 2001 Explorer Sportsman Concept Looks For a New Home
Verdad Gallardo
10 Best Ford Truck Engines We Miss the Most!
Joe Kucinski
2026 Shelby F-150 Off-Road: Better Than a Raptor R?
Brett Foote
2027 Super Duty Carhartt Package First Look: 12 Things You NEED to Know!
Michael S. Palmer
10 Most Surprising 2026 Ford Truck Features!
Joe Kucinski
Cargo Master
Joined: Mar 2002
Posts: 3,132
Likes: 218
From: W (BY GOD) V
LET'S START FROM THE BEGINNING AS ALL OF THIS HAS BECOME OVERLY CONFUSING
The truck in question is a 1969 F250 2WD that was assembled as a manual brake (no power booster) from the factory?.
Somewhere along the line it was decided to add a booster?
This entails replacing the MC as MANUAL and POWER MC's are different (bore sizes).
AM I CORRECT IN MY ASSUMPTIONS SO FAR?
The post about the DISC/DRUM question threw a curve ball ...
Last edited by KULTULZ; Apr 3, 2026 at 12:15 PM.
Mountain Pass
Joined: Nov 2012
Posts: 109
Likes: 10
From: Hurst,Texas
K. Adjusted the rear brake shoes they were off a little then had the little wifey step on the brake to see if the wheel cylinders are working both are working. So set the truck back down on all four and put it in reverse and slowly let off the brake and about even with the throttle pedal it begins to creep backwards so I’m pretty happy with that I’ll keep an eye on it. Thanx for all the help and input on this matter. After rims and tires the next upgrade will be rear disc brake kit from LMC
Last edited by DCrusher; Apr 3, 2026 at 05:00 PM.
Mountain Pass
Joined: Nov 2012
Posts: 109
Likes: 10
From: Hurst,Texas
So I’ll be the only one test driving and checking it out.
Cargo Master
Joined: Mar 2002
Posts: 3,132
Likes: 218
From: W (BY GOD) V
It is her health I am worried about, not the truck. Just make sure it stops correctly especially in a full panic stop, before you give it to her (and yeah, they always win)
She sounds like a good woman. Lucky man.
She sounds like a good woman. Lucky man.