Topic: Brake pedal travel: Residual Pressure Valve to the rescue?

Our Mustang II still proudly wears rear drum brakes but over the course of a day we have to pump them every time. At the end of the day we can adjust them and they'll be ready for the next day, but only for a while. Eventually we have to pump, pump, brake! Its bad enough that for days after a race I'll pump the brakes on my daily driver out of habit.

Rather than adjusting them midday (or braking in reverse around the paddock to auto-adjust), can I limit the pedal travel by installing a residual pressure valve on the rear circuit? I read online that most master cylinders have one built-in, but ours is newish and I'd assume it kinda/might/maybe has one if it's supposed to be there.

I think I understand how they work, but is this the right approach? Will a 10lb RPV solve the problem or just make my shoes drag and wear? Will it lead to the wheel cylinders getting over-extended and blowing out?

Low Road Racing:  Stylemaster/Mustang II/Sunbird/Spirit AMX

We will not compromise our morals and build anything but a class C car.
"You take the high road, we'll take the low."

Re: Brake pedal travel: Residual Pressure Valve to the rescue?

Disclaimer, this is a stupid idea:

How about installing the factory emergency brake set up including the handle? With it you can adjust as you go.


Bill

2020 I.O.E. CT #36 The Rootes Of All Evil,1958 Sunbeam Rapier Convertible (YES 1958!!) & 2019 Judges Choice NJMP
2016 Thompson Speedway #36 Sabrina Duncan's Revenge, IOE Trophy, 5th Place 'C' Class 1977 Ford Pinto
2009 Stafford Motor Speedway #16 Team Teflon, 11th Place (overall) 1997 Saturn SL2

Re: Brake pedal travel: Residual Pressure Valve to the rescue?

And what is going on with the front calipers while you are pumping the pedal to engage the rear drums? My gut reaction is you have a bigger issue. Most of your braking power should be coming from those front calipers without pumping the pedal. If you are pumping to a point of feeling the rears engage the fronts should be locking up. Some things to look at:
- front caliper bleed valves should be at the top of the caliper (not bottom). If they are at the bottom you have the right and left sides swapped.
- check the master cylinder part number and make sure somebody hasn't installed one with a too small bore size.
- if you sit with foot hard on the brake pedal does the pedal slowly sink (pedal to master cyl shaft misadjusted or master cyl seals leaking)
- check the bleed at each wheel for a good flow, you may need some new brake lines.

Re: Brake pedal travel: Residual Pressure Valve to the rescue?

I can tell you I know the symptoms that you speak of. You've actually got pedal with the first push, but it is so far down at the limit of travel that it is unsettling. If you double tap it raises the pedal and you feel more confident and it makes blip throttle downshifting much much easier.

I've done the hand brake thing. Sometimes helped.

The solution for us at the time was to block off the rear brakes and run with fronts only... Not that I'm suggesting this for YOU or anything. But an underpowered 1200-1300 lb car with no rear drum brakes isn't really an issue.

I don't know if 10lb valve is too much, but from instinct it sounds like it might be. I'd try a 2lb one like the vw bug guys use

Re: Brake pedal travel: Residual Pressure Valve to the rescue?

This symptom is common with front disc/rear drums cars when used in "anger" I had the same issues back in the '80 with my Fox Mustang that I open tracked and even on my still earlier '74 Mercury Capri! 
I would use the check valve in line to the rear brakes but be aware that you may experience higher brake shoe wear. How much is a guess. They may last a few races or they may wear out after only one but if the change gives your drivers the confidence to drive the car faster then I think it's worth it for the cost of brake shoe and the valve.

I installed the Wilwood valves on one of my race cars on the front brakes when I experience "pad kickback" and an inconsistent pedal.
It fixed most of that issue but not 100%.

6 (edited by chaase 2019-06-16 09:00 AM)

Re: Brake pedal travel: Residual Pressure Valve to the rescue?

Spank wrote:

I can tell you I know the symptoms that you speak of. You've actually got pedal with the first push, but it is so far down at the limit of travel that it is unsettling. If you double tap it raises the pedal and you feel more confident and it makes blip throttle downshifting much much easier.

I've done the hand brake thing. Sometimes helped.

The solution for us at the time was to block off the rear brakes and run with fronts only... Not that I'm suggesting this for YOU or anything. But an underpowered 1200-1300 lb car with no rear drum brakes isn't really an issue.

I don't know if 10lb valve is too much, but from instinct it sounds like it might be. I'd try a 2lb one like the vw bug guys use

10lb is the recommend  pressure for a residual valve for a drum setup. They require more pressure than discs because there are springs pulling the pads back on a drum setup so it needs more pressure.  We use the 2lb in our other cars but they are all 4 wheel disk

1992 Saturn SL2 (retired) - Elmo's Revenge -  Class B winner, Heroic Fix winner x2
1969 Rover P6B 3500S(sold) - Super G-Rover - I.O.E Winner, Class C Winner
1996 Saturn SW2 - Elmo's Revenge (reborn!), Saturn SL1  Dazzleshipm Class C x2 and IOE winner
1974 AMC Javelin - Oscar's Trash heap - IOE,”Organizer's Choice" and "I got Screwed" award winner

Re: Brake pedal travel: Residual Pressure Valve to the rescue?

Being the guy that "brakes are my #1 priority"
the answer is, for me, drums have no place in endurance racing.  We do EVERYTHING IN OUR POWER to avoid them unless it is an IoE build...and we still disk swapped two out of three of those.  There is always a low-cost option.

It is not that I think drum cannot be effect because I know they truly can.  the maintenance issues plus potential problems jut make me say..no to drum brakes.

Re: Brake pedal travel: Residual Pressure Valve to the rescue?

As a auto tech:
E brake handle may work

As a racer:
Higher quality rear shoe with racing compound may wear slower.

Note:
No actual drum/track experience

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