Residual valves are only used on drum brakes, in a disk drum set up the proportioning valve portion of the combination valve may do double duty acting as residual valve and a proportioning valve.

The metering valve on the disk circuit can take the place of the residual valve allowing the drums to take up the slack before the disks are applied but generally the 2 are used in conjunction with each other. In the old single circuit Drum drum set up the residual valve was in the master cylinder and called the check valve in Ford terminology.

The Residual valves, do double duty it keeps about 10PSI in the system so the shoes do not have as much take up till brake application. In under floor master cylinder applications where the master cylinder is lower than the wheel cylinders the pressure keeps the cup seals sealed against the cylinders walls. With out it the brake fluid could drain back to the master cylinder as cup seals only seal in one direction and this would allow air to intrude in to the system.

Disk brakes do not need residual valves unless used in a underfloor master cylinder application then it advisable to install a lower pressure (2lb) residual valve.

If the master cylinder fails either through wear or corrosion the combination valve should also be replaced as a matter of fact to maintain optimum braking efficiency.
Residual valve operation and in turn combination valve operation can easily be checked if it's operation is in doubt.

Really in these old systems any time multiple major brake hydraulic components are replaced an up date to DOT 5 fluid should be considered this does mean flushing the system but once done and filled with Dot 5 the system is life time and likely will never need another hydraulic component replaced. This comes from experience with Dot 5 system that are 30 plus years old and still functional as new the original hydraulic components.
DOT 5 should not be used in ABS equipped systems.

 

The truck has drum brakes and requires a residual valve on that circuit, whether free-standing or incorporated into the proportioning valve.

Only if the components are compatible with DOT 5 fluid.

 

Thanks for stating again what was already said.

As for compatibility I love how this gets tossed around by people with out even knowing what DOT 5 is not compatible with.

All brake components manufactured or rebuilt by companies that adhere to the Federal Motor Vehicle Safety Standards have been DOT 5 compatible since 1971. Silicone fluid is not compatible with natural rubber seals. In fact natural rubber seals are not compatible with Dot 4 or 5.1 fluid either. For the most part they quit using natural rubber seals in in braking systems in the 1950's. All brake components ,seals and hoses whether new or re-manufactured have had to conform to FMVSS 116 since 1971 which includes Dot 5 compatibility.

So compatibility is not even on the radar in this application.
And the only time it would even be a consideration is if a person was using NOS bits that date prior to 1971. Most vehicles that age and older have long since had all their brake bits replaced.

This Dot 5 compatibility thing dates to the late 60's's when it first came on the market and was found to soften and swell natural rubber seals when existing mil vehicles were switched over to it. This is also what prompted the creation of the 116 brake fluid and compatibly standards in the first place.

DOT 3, 4 and 5.1 Brake fluid should be checked annually with a brake fluid moisture tester, Dot 3 and 4 should be flushed every 24 months and Dot 5.1 every 36 months Dot 5.1 is not as hygroscopic as 3 and 4 so the flush interval can be extended, that is why basically all new vehicles used 5.1. Dot 5 on the other hand is a life time install and never needs to be flushed or checked as it does not absorb water nor does it hurt paint, or corrode parts when exposed to the atmosphere. When renewing the brake system in any non ABS vehicle not using DOT 5 is just stupid.

 

Eh, DOT 5 can be be a real pita from what I understand. It's a bitch to get all the bubbles out if not very careful when filling and it is even compressible to some degree, leading to soft pedal complaints. It was developed for military use originally. Glycol based brake fluids absorb moisture, it is true. This is kind of a good thing in the sense that accumulated moisture would otherwise pool and freeze solid in cold weather. It does lower the boiling point over time.

What I never understood is why a completely sealed system wasn't used, with hydraulic fluid, as aircraft do. ATF would be about perfect. Aren't their petroleum tolerant rubber parts in transmissions? I like good ole DOT 3 - believe it or not DOT 4 is actually inferior in some respects.

 

The compressabilty is only noticeable in high performance brake or manual systems in terms of feed back. And even then it only translates to a slightly softer pedal. In a power brake equipped production vehicle you can not tell the difference. 99.5% of people could not tell the difference even in a manual system. Getting the air out is not big deal and just requires a second bleeding if done manually if power bleed it is not necessary.

Funny how these things come up but ask to find person that has actually had these issues it all comes up blank. I've got over 30 years in on a couple vehicles with DOT 5 with no issues that's longer than some of the posters on this forum are old. I have never had to replace a single hydraulic component on any of them in that time. Plus Dot 5 is non toxic unlike Dot 3,4 and 5.1

H-D uses Dot 5 from the factory as does Honda in some of their bikes the Military uses it in all equipment including MICOTS (MIlitary Commercial Off The Shelf) vehicles.

Some manufacturers did use mineral hydraulic type fluids, LHM fluid to be precises. it was used by Citroen Peugeot, Mercedes, Rolls Royce, Jaguar, Bentley and a couple others.

LHM does not absorb water either but it has more air entrapment and in turn more comprehensibility , just like Dot 5.
Since LHM aerates easier than conventional brake fluids it is not suitable for use in Anti-lock brakes again just like Dot 5. In Aircraft the braking systems are power pressurized and not power assist and use much higher pressures and volumes so the comprehensibility thing is not an issue.

LHM is also used as shock oil and as the brake fluid in high end bicycles and as a hydraulic fluid in some tractors.

And I would be interested in hearing in what aspects you think Dot 4 is inferior to Dot 3.

 

Harley doesn't use DOT 5 anymore as I understand it. DOT 4 is technically superior stuff, at least when new - over time though its boiling point falls off even faster than DOT 3. I just buy a high quality DOT 3 - not the cheap stuff at the drug store - and flush it every couple years. Yes it depends on the application. Older classics with juice brakes, manual drum systems, for topping off and maintaining them it's fine. You're right, the stuff in this forum is probably too new. So in that narrow sense - fresh high quality 3 will have a higher boiling point than 4 in a system that hasn't been flushed in a long time - and that's usually the case in this country. Lots of cars and trucks never get any brake attention whatsoever, except maybe pads once in a while. I try not to get too spooled up on this stuff. With a completely rebuilt brake system DOT 5 may be the way to go, but for older classics I think DOT 3 or 4 is absolutely fine.

 

I think you have gotten some bad information somewhere or go them mixed up .
DOT 3 fluids are only glycol based, they have a higher rate of hygroscopy than DOT 4 fluids that are borate ester/glycol based, not only does DOT 3 start out with a lower boiling temp it falls off faster and further than DOT 4 when contaminated.
DOT 4 is formulated to higher specification and was designed specifically to address the some the issues with DOT 3 , not just in boiling point but also in regards to hygroscopy and better maintaining of the boiling point when contaminated with moisture.
So you got some bad info there.

The life expectancy of cast iron brake components (master cylinder, calipers, slave cylinders) with conventional brake fluids is about 8 years some go much longer others much less but 8 is the average. With DOT 5 it is basically unlimited.
To fully redo a non ABS brake system then put a conventional brake fluid in it is sheer lunacy. As the system is degrading and rusting from day one with conventional brake fluids.

Harley switched to DOT 4 when they went ABS, prior they had been using DOT 5 since the mid 70's.

 

Keep in mind there are minimum standards for boiling point that a product has to meet. Quality DOT 3 exceeds those minimum standards by quite a bit. It's my understanding that fresh good quality DOT 3 in practice will have a higher boiling point than old DOT 4 when used in the relatively primitive braking systems from years ago. It's probably not going to make any difference, but it's interesting.

DOT 3 is a thinner viscosity compared to DOT 4 and bleeds easily - very important for DIY types - and seems to work well in cold weather. DOT 4 was marketed as "racing" brake fluid at first - specifically for high temperatures. People pushing the envelope tend to be meticulous, or should be, and don't leave brake fluid around long enough to become contaminated. See where I'm goin' with that?
----------
http://www.stoptech.com/technical-su...rs/brake-fluid

"DOT 4 fluids are also glycol ether based, but have a measure of borate esters added for improved properties including increased dry and wet boiling points. A seldom talked about characteristic is that because of this chemistry, the DOT 4 fluid will have a more stable and higher boiling point during the early portion of its life, but ironically once the fluid does actually begin to absorb water its boiling point will typically fall off more rapidly than a typical DOT 3. By FMVSS116 standards, DOT 4 fluids must have a minimum dry boiling point of 446°F and a minimum wet boiling point of 311°F. DOT 4 is the grade applicable to most race engineered brake fluid in the world today, especially with regard to viscosity limit. Note that although the DOT 4 designation has a minimum dry and wet boiling point, a DOT 4 racing brake fluid may have a dry boiling point over 600F. Its viscosity is challenged, however, to be under the viscosity limit of 1,800 mm2/sec. Some claimed racing brake fluids exceed this important limit. Caution should be exercised if these fluids are used in race cars with ABS systems. This does not mean that DOT 4 fluids are necessarily better than DOT 3 fluids. Remember, the boiling points listed are minimums. There are certain DOT 3 fluids with higher boiling points than some DOT 4 fluids. The real differentiating factor is that DOT 4 fluid should be changed more often than a DOT 3 fluid, because of the effects and rates of water absorption."

Hope this helps.

 

Well - like most things - "it depends". For vintage - real vintage - it has all the right things going for it. Never needs to be changed. Sounds good to me, but the rubber components in the M/C and wheel cylinders and hoses had better be silicone compatible or it's going to be a real mess. See where I'm goin' with that? From what I've read the classic owners really like the stuff - but there are also a lot of users who complain about air bubbles/foaming and bleeding problems, soft pedal etc. It's good stuff but it's more complicated than "Well I'll buy the best" a little planning is involved to make sure it's done right.

 

You can't compare new vs old that is like saying new bias ply tires are better than bald radials.

And Dot 4 was initially used in systems that generated more heat not in race applications. Just as we now have 5.1. DOT 4+ is the racing version of DOT 4

Dot 4 is NOT as hygroscopic as Dot 3 so no it does not fall off as fast as DOT 3 nor does it's boiling point ever fall below DOT 3 when equivalently wet. ALL equvelent Dot 4's are better than any equivalent DOT 3. Plus performance DOT 3's are pretty much obsolete there are still few manufactures that still make Performance DOT 3's but they are few and far between . Additionally DOT 4 systems are not backwards compatible to DOT 3 usage. But Dot 3 systems can use DOT 4.


See bottom chart right from Shell in regards to boiling points in DOT4 's and DOT 3 with moisture contamination. As was stated before 2 years is the flush interval with conventional brake fluids.

The DOT 4's out perform DOT 3's in all circumstances even with twice as much water content DOT 4 still has an equivalent or higher boiling point than DOT 3. So no DOT 4 that has been in service twice as long as DOT 3 is not less effective than DOT 3. Dot 4's only draw back is it is it has a thicker viscosity at sub zero temps and even then it is minimal.


Dot 4 ESL (extended service life)
Is an extended service fluid that will extend your flush interval to 30- 36 months

DOT 4 Ultra/ +/ Super is the racing version of DOT 4 it is a lower viscosity brake fluid lower than 3 or 4 and with a lower rate of moisture absorption.

See below for viscosity of the common brake fluids.

DOT 3
Temp. [°C] Viscosity [mm²/s]
-40 1500
100 1.5

DOT 4
Temp. [°C] Viscosity [mm²/s]
-40 1800
100 1.5

DOT 4+, Super, Ultra
Temp. [°C] Viscosity [mm²/s]
-40 750
100 1.5

DOT 5.1
Temp. [°C] Viscosity [mm²/s]
-40 900
100 1.5

DOT 5 (MIL-PRF-46176)
Temp. [°C] Viscosity [mm²/s]
-55 900
100 1.3

LHM (PSA B-712710) (Mineral)

Temp. [°C] Viscosity [mm²/s]
-55 1200
100 6.5

Again you have gotten some erroneous information from somewhere at some time.

 

Again any brake components built in NA after 1971 will be DOT 5 compatible.
As said before this is only a concern if you are using old NOS bits with natural rubbers in them. And this does not apply to either the dent sides or the bull noses.
And bleeding is no different than regular brake fluid with a couple caveats, let the fluid stand for an hour before adding from the container so any air bubbles can settle up an out, (do not add after shaking it) and do a couple follow up bleeds, which is what most people do any way with regular fluid..
Fail to do this and you very may have issues. Follow these simple rules and you will have no issues. Small price to pay to not have to flush the brakes system every 2 years.

As for compressibility differences, even at 400°F the compressibility difference between DOT 5 and DOT 3 is less than 2% (See below chart). In 90% of automotive power brake system you will never even be able to tell the difference. You certainly will not be able to tell the difference in a Bull Nose. You may feel a difference with manual brakes but it will just mean a minutely softer pedal which in it's self is not a bad thing in a non power braked vehicle.

 

Seems to me this is pretty well established, that's why I brought it up in the first place. I don't believe it is erroneous information.

My point was really, it's easy to get waaay off into the weeds on this stuff. It's not like the ditches are filled with cars and trucks that use DOT 3, hell - millions of cars are driving around with brake fluid that's 20 years old. It's not good, I'm not saying that at all. It's just easy to slip into silliness over this. There is absolutely nothing wrong with DOT 3 brake fluid, and depending on application it has some advantages in some respects.


"If, water is absorbed into DOT4 the boiling point drops rapidly, even more than DOT3. DOT4 is referred to as a race or performance brake fluid because of the viscosity limit. Therefore when it comes to performance vehicle it is important to change your brake fluid more frequently."

https://www.autodeets.com/top-5-what...t-brake-fluid/

 

..........

 

One more:

"When deciding which brake fluid to purchase, the best advice is to stick with what’s in your system. Just because a fluid falls within a certain classification doesn’t mean it won’t provide equal or better performance than a fluid with a higher classification. Depending on the composition, a DOT 3 fluid may have a better boiling point than a DOT 4 fluid. In addition, vehicles with an anti-lock braking system (ABS) are designed to work within the viscosity range of the specified classification."

http://www.machinerylubrication.com/...w-brake-fluids

 

This article is in error DOT 4+ is the race spec DOT 4 fluid as it has lower viscosity than DOT 3 regular DOT4 actually has a higher viscosity than DOT 3 So that is factually incorrect. Additionally DOT 4 has an equivalent and lower rate of boiling temp drop when it absorbs water again refer to the chart from the manufacturer. This article has blatant factual errors and he does not seem to know the difference between DOT 4 Dot 4 ESL or DOT 4 + So the old saying "don't believe everything you read on the internet" applies here.