Residual Valve Question - Help Please

[Motomazzo]

Probably a dumb question, but I have to ask:

I was just researching old threads looking for answers to the purpose of a residual valve on the rear brakes when installing 4-wheel disc brakes. There were mixed answers all around. I just spent a lot of time/energy this weekend, installing a 2-lb residual valve on my rear brake line. Here's what I have:

New VW Dual M/C, stock, without residual valves on any of the outlet ports
4 wheel Disc Brakes
2-lb residual valve installed in rear brake line
(This is a project car and I cannot road test at this time)

Is this correct?

I’ve read that some say that disc brakes must NOT have a residual pressure valve in the circuit. Others are pretty consistent in saying that a residual valve on their rear disc brakes works great and helps them with a firm pedal. Evidently, CB disc brake kits highly recommends one and sells them…..and I don’t see what would be different about any other brand of disc brake kit.

To be honest, in a closed circuit hydraulic system like in a brake system, when the pedal is not being depressed.....I can't see how bleed back could occur. Maybe someone can help me understand this?

Thanks for your time!

Moto

[61SNRF]

I believe the main reason residual pressure valves are installed in drum brakes is to pre-charge the W/C's because drum brakes have strong return springs that would take a lot of pedal travel to overcome. They can also prevent water entry into the fluid. Under a slight pressure it resists the absorption of airborne H2O. They are normally about 7-12 psi.
Disc brakes don't need them this high as they would drag and wear quickly. They usually have about 2 psi on most cars by the act of gravity, the M/C being higher than the calipers. In some cases (like yours) where the M/C is lower than the brake caliper, they are used to maintain about 2 psi. If mfr says to use them then I would do it.
Additional reading here...
http://www.thebrakeman.com/valvetechi

[Motomazzo]

Thanks for the info, 61. I'll have to read that and learn some more. The manufacturer (EMPI) did not say to use a residual valve....at least I don't think so. I will have another look through the paperwork. But I read many posts where folks said to use a 2lb on the rear line on any 4-wheel disc set-up. I was talking to my father about the valve the other day and he's the one that said that was rubbish. I started to defend the valve….but couldn’t really come up with anything, LOL! After I started thinking about it, I couldn't imagine how there would be a bleed-back and any need for a residual valve. When I started digging, I read mixed information. That's why I decided to start a thread to get some discussion going.

Thanks again!

Moto

[Teeroy]

I did 4 wheel CB brakes with an Oldspeed modified bus master, and needed the 2lb residual valve on the rear line.
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[Motomazzo]

Thanks, Teeroy!!! So, when you say you "needed" the residual valve....what do you mean? Was the pedal travel ridiculous until you put the valve in? Also, I noticed in doing thread research that you are using a single-circuit M/C with your 4-wheel disc set-up. Is this why you needed a residual valve?....as opposed to a dual-circuit M/C. Again, this would really be no big deal if I wasn't doing a body-off. It may be a while before I can road-test. I am doing the "plumbing” now and just want to be done with it and know that I did it right. Actually....it's already been plumbed. I guess the only thing I would need to do is remove the valve if I got an overwhelming response to do so. Not too many opinions on the subject so I may just leave it as is and road test when the car is done. I do appreciate your input, Sir. Thank you.

Moto

[drscope]

Residual valves are NOT needed with a 4 wheel disc set up. They ARE needed with a front disc, rear drum set up.

What they do is to keep a small amount of pressure in the line so the shoes can not retract all the way back from the pressure of the brake return springs.

If they were allowed to move all the way back, you would have to pump the pedal each time you wanted to stop in order to pump the rear shoes out tight against the drum.

With a disc set up, the piston does not really retract. It stays out close to the rotor. So there is no need to have residual pressure valves in a 4 wheel disc set up.
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[Teeroy]

I did not have very good pedal and the rears did not work properly until I added the valve. I did this ten years ago and all is still working like new, still have the same set of pads on all four corners, so the valve has not caused any extra wear on the rear set.
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[Motomazzo]

This is all starting to come together now. I see where you would need one for drum brakes and how the bleed back is induced by the shoe spring, pulling the shoes back in and collapsing the slave cylinder.

Dr. Scope - On the other hand from what I've read so far, this also seems to be one of those things where there is a noticeable disparity between theory and reality. I've read many, many posts from guys that said they had too much travel in the pedal (on a 4-wheel disc system) before they installed the residual valve. Like Teeroy here for one. No real good explanation...just one of those black magic things. CB even states that one is required.

I'm wondering if it has something to do with the e-brake set-up. I know from taking mine apart there is a cam and a pin that actuates the piston when the handbrake is pulled. Maybe there is some type of "slack" in the system from this?? A reach, I know. I'm just trying to figure why you would need one in a disc brake set up. Evidently, you do.

I think what I will do is leave my residual valve installed and monitor the rear brake wear, etc. From what I read, I'm gonna need one to get a good firm pedal.

P.S. - I've read boat loads of posts recently from folks installing their full-house disc systems and complaining of a spongy pedal. I'll bet you some of these are mistaking a spongy pedal for too much travel and they need a residual valve like many have said.

Moto

[Motomazzo]

Just an update to this old thread I started......

I've been driving my chassis around quite a bit. I do not have a spongy pedal and in fact....it's about as solid of a pedal as you could ask for. I've been really worried since I installed the residual valve on my 4-disc system that my rear brakes would constantly ride the rotors.....more so than what is normal. But so far, that has not been the case.

Having said that, I am going to do some brake line maintenance for some other reasons. I am considering taking the residual valve out of the circuit. If I do and it makes a change for the worse, I will definitely update to let everyone know.

Moto
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[jhoefer]

Two reasons to have 2 lb residual valves on a disc brake system: drain back and knock back. With the master cylinder mounted so low, the calipers could end up being above the MC/reservoir and fluid could drain or siphon back out of the calipers. Can also cause problems with brake bleeding. If there is any runout in the brake disc, it will knock the caliper piston back into the bore. It makes the gap between pad and disc wider than normal and this extra travel needed by the piston is amplified as a dead zone at the pedal .

Are they required on a disc brake system? No. But using them can only help.

Drum brakes definitely require 10 lb valves to keep the springs from pulling the shoes away from the drum and to keep the wheel cylinder seals from leaking. However, most OEM drum brake MC's have 10 lb valves built in. Disk brake and and most aftermarket MC's don't have valves built in. Just don't add an external valve to a MC with built in valves, the brakes will lock up.


Good luck with your test.

[Deanodynosaurs]

This is a really interesting topic.

I have fitted CB rotohub disks at the back of my 58 beetle, and a later beetle disks setup with talbot horizon callipers at the front. I'm also using a 'performance' master cylinder from SSP. I have literally just bleed it all tonight and been round the system a few times. However there is still quite a bit of travel in the pedals.

Now I know the pads, disks etc need to bed in, but I wouldn’t expect as much travel as there is. Don’t get me wrong, it’s not going to the floor, but there is excess travel. However, I’m not gonna get the chance to put a few miles on the brakes, as it a trial build I’m doing.

Thus does this sound like a case where I’d need to add a residual pressure valve?

Also, Mottomazzo, did you take the residual valve out your system? Did it affect pedal travel?

I look forward to hearing what everybody has to say.

Dude
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[drscope]

You didn't mention what your pedal free play is. You may simple need to adjust your pedal stop or the length of your push rod.
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[Deanodynosaurs]

[bluntlunchbox]

Hey had to revive this thread to ask a question. I have 4 wheel discs and don't seem to get a lot of bite when I push down on the pedal. I've bled the heck out of this thing but feels like I have to push way too hard than I should. It's almost like the front doesn't grab enough, no dive when I hit the brakes hard. The braking performance works the same whether I use the E-brake or push the pedal. Would a residual valve help me?

[Deanodynosaurs]

Hi bluntlunchbox,

Diffrent people seem to have diffrent opinions on this.

Firstly what master cylinder you using? Is it one specfically for 4 wheel disk brakes?

If so, and you still got problems, from the research ive done there could be to solutions.

1) As you suggested one solution is a residual pressure valve. Only a very light one, 2lb, as if not the calipers will stay locked on. This goes in the line before it reaches the t piece at the back.

2) Heres what i had sucsess with (And i should have posted this up before). I fitted a 944 master cylinder (not with a residuel pressure vavle). This really sharpened my brakes up. Only down side for me was i have a RHD car, and he brake lines come out right next to the tunnel. I'm guessing yours is LHD so it should be fine. The rationally for the larger master cylinder was that for me the travel on the pedal was excessive, and it did not bite quickly, which i attributed to the master cylinder having to transfer a lot of fluid to operate the larger calipers. Thus i increasesd the size of the master cylinder, thus the fluid in it, and less travel to operate the calipers. Dont know if the mechanic of this are fully correct, but it worked for me.

However, as for your problem i'm a little confused. On my rear brake the e brake is mechanical, so that worked well even though pedal was squidgy. Does your e brake work hydralically of mechanically. If its mechancially there something wrong with how its set up.

Hope that helps.

Dude
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Dude's Beetle Webpage (VWPRO70)

[bluntlunchbox]

Thanks for the reply! I have the 4 wheel disc brake master from EMPI, along with EMPI discs at all four corners. The e-brake is mechanical. The braking sensation I experience I feel is the same whether I push the pedal or pull the handle. Almost as if I'm not getting enough bite in the front. I was curious if the residual valve was needed to fix that.

[Chalklinedetail]