Brake Hydraulics Questions

[JeffYoung]

In the never ending quest to make the crap brakes on my car (discs in front, drums in back) work, I wanted to confirm with you guys some information I discovered last weekend.

Most stock disc/drum brake setups had what is called a "residual" valve holding 10 lbs of pressure, either located in a separate place in the rear circuit, or as part of the stock proportioning valve. The residual valves provided just enough pressure to couteract the drum springs and push the shoes out against the drums when the brakes were not activated. This gave a firm pedal feel, as without the valve, the shoes would retract slightly due to the springs and the pedal would be soft as a result.

When I first built the car, it had decent pedal feel. I then took the stock proportioning valve out of my car, and replaced it with an adjustable. I believe the stock proportioning valve had a residual valve in it.

After replacement, I've never been able to get a truly firm pedal. It appears to me the answer to this is a residual valve?

Do the guys with drums in the rear run these valves? I'm plumbing one in over the weekend on the theory that brake circuitry is free and may be revised, and was part of the stock system anyway.

[Greg Amy]

Jeff, that sounds like a real oddball (surprise!) Most vehicles I've worked on for the last 20 years with drum brakes have some kind of manual or automatic MECHANICAL (not hydraulic) method of limiting drum shoe pullback... - GA

[JeffYoung]

Thanks Greg -- inside the drum itself? My car apparently does not.

The literature I read indicated that most cars that had a proportioning valve on a disc/drum setup usually had a residual valve in the stock prop valve. 240Z guys,was this true with your cars as well?

[tom_sprecher]

I thought the brake adjuster when actuated by applying the brakes while in reverse is what kept the shoes close to the drum as the linings wore. That is what prevented a soft pedal.

[Greg Amy]

In "the early days" (i.e., before the mid- to late-70's or so) the shoes would pivot on one end and the other end was actuated by the slave cylinder. Somewhat down from the cylinder end was a tie-bar with a threaded rod in the middle, such that you would use a screwdriver to lengthen or shorten the bar to set the shoes to just off of touching the drums. That bar is what kept the shoes from pulling back too far.

Starting around the mid-70s I started seeing automatic adjusters (first time was on a Rabbit.) Same idea except there was a spring-loaded triangle-shaped wedge that fit in the middle of that bar. Instead of having to manually adjust the length of the bar, as the shoes wore this wedge would get pulled down further, forcing the ends of the bar apart. When you braked in reverse the forced were such that the bar pulled apart and allowed this wedge to work its way down and hold it there. Thus, no more manual adjustment.

I've personally never worked on a car with any kind of hydraulic hold like you mention. Of course, I don't work on cars for a living, and the British have designed things far more odd than that, so I don't doubt it exists. But it sure seems pointless... - GA

[ddewhurst]

The info Greg presents is as I know the drum rears to be. Jeff, ask your same question on the Production site where there are more Britrish using folks.

[seckerich]

Some of us converted production guys lurk here as well!! If the rear is like the Spitfire brakes it will just have the slave and a bar to seperate the other end. We ran a 2 lb residual valve. As I remember we had to run the Wilwood inline when we had the rear drums on the GRM rotary spit until we went with the disk setup. It was a 2lb also and was plenty to keep pedal feel without dragging the shoes and heating the rears up.

[tom_sprecher]

It may have been the mid 70's for foreign cars but self adjusting drum brakes have been used on domestic cars since the mid 50's. Come to think of it my '72 Capri had self adjusting brakes as well. Maybe it was only the Brits that were slow to accept them.

[Roy Dean]

Jeff, that sounds like a real oddball (surprise!) Most vehicles I've worked on for the last 20 years with drum brakes have some kind of manual or automatic MECHANICAL (not hydraulic) method of limiting drum shoe pullback... - GA
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But thats only at the end opposite the slave, what about the slave side? Both sides of the shoes must be in continuous adjustment.....

[Greg Amy]

No, that bar *IS* on the slave side. The opposite side of the shoes on most drum systems is a fixed pivot point...

[JeffYoung]

Guys, many thanks for the information.

Yes, the car only has the wheel cylinder at the bottom of the drum. At the top, the shoes just shit on a metal peg and pivot on that.

It does have a toothed adjuster that "clicks" the shoes out over time, but in between teeth there is a fair amount of travel.

I've plumbed in the 10 lb based on Wilwood's recommendation to use that for the drums. Maybe with larger drums inthe rear than on Steve's spit, and larger springs, you need the additional pressure. We'll see. Easy fix to convert to 2 lbs if the brakes drag.

Thanks again guys. Headed to the SIC next weekend; I'll let you know if the pedal feel is any better.

[tom_sprecher]

This is a good representation of what all the drum brakes I've worked on look like and the animation is kind of informative as well.

Drum Brake (http://auto.howstuffworks.com/drum-brake1.htm)

After watching it the pivot does not act so much as a pivot but more as a "stop" with the slave on one end of the shoes and the adjuster on the opposite end.

What's interesting is if you watch the animation you can see how the shoes "wedge" into the drum which in effect gives additional braking force.

You can learn something new everyday.

[zracre]

In my 240Z, at most tracks I had rear-shoe-on-the-fly-adjustable brakes....just pull up the e-brake one click per lap if it was a hard on brakes type of track...

now that was a cool website...they even have the brake adjuster animation if you page down

[JeffYoung]

Interesting. I'm certainly no expert on drum brakes, but it appears that the TR8's are -- imagine this -- quite archaic. The wheel cylinder (slave) is at the bottom of the drum. At the top is a metal stud around which the drums pivot (in a sense). There is no star wheel and adjusting rod like on the 260z we have, and in the animation (thanks for that Tom, nice find).

There is a toothed piece of metal, actually two, whose function appears to be to push the drums out as they wear. It is activated when you step on the pedal. If you take the drum off, you can push the pedal in and watch the toothed metal try to advance to the next tooth. In between teeth there is quite a bit of play.

I suspect that a residual valve of some sort (will try 10 lbs first per Wilwood) is necessary to push the shoes out and counteract the springs "in between" gear teeth.

Interesting.

Thanks for everything guys.

Jeff