Technical Help with Brake Combination Valve Question

Based on several adjustable PVs I've worked with over many years.....
The minimum 100 psi and maximum 1000 (3000??) psi valve settings are the "crack or knee" pressure points where the 57% proportioning begins. The normal 10-turn adjustment moves the knee between these extremes.
The 57 % spec, one of many (.27, .43 are a couple common others from memory) used by the OEMs is the amount of pressure added to the master cylinder output, after the knee is reached.
So, with a 500 psi M/C input and a 100 psi valve setting, the rear brakes will receive 100 plus 57% of 400 (228) for a 328 psi. total. With a 1000 psi M/C input and a 500 psi valve setting, rears will receive 500 plus 57% of 500 for a 785 psi total.

 

You really don't need RPV's ...

 

Let's take this in pieces...

Pre-'67 Ford metering valve. Got a parts diagram showing one? There's plenty of references of the '65-66 Mustang guys searching for the long-discontinued OEM proportioning valve for these when doing a restore, but none for a metering valve.

The Jags I referenced are four wheel disc brakes (4WDB). Jag ditched drums for all discs in the '60s. All the references you find for metering valves 'explains' their use as allowing the car to 'settle' evenly when applying the brakes on a disc/drum system. Which makes the inclusion of a front delay on the Jag 4WDB systems even more puzzling.

"Someone needs to pull out an accurate textbook and read the purpose/need for a METERING VALVE on a DISC/DRUM setup...."

Seeing how YOU brought this up, please do show this info.

Again, I find it very interesting that both Baer and Wilwood don't see any need to offer a metering valve. While they offer 4WDB systems as the core of their business, they both also offer front-only conversion kits where supposedly these would be needed.

Where these OEM-style combination valves are prevalent is in the 'lower priced' aftermarket kits (although many of these kits are more expensive than the higher-end Baer or Wilwood kits). These kits re-task OEM bits and IMO they 'require' these valves simply to mimic the OEMs without understanding the need or lack of it for specific applications. The aftermarket is quite adept at jumping onto bandwagons, I see this as another case of it. I suspect that legal liability ramifications may be the biggest influence. As long as all the same bits are present like the system the parts were sourced from, it must be 'OK'...

Again, I'm not claiming these would be detrimental if installed, I'm questioning if they're actually needed.

 

Hopefully, this will put to rest the questions about metering. I will state from a '74 Bendix brake system service manual.
Metering Valves
"Some automobiles equipped with front wheel disc brakes and rear wheel drum brakes have a metering valve installed in the hydraulic line to the disc brakes.
The basic function of a metering valve is to improve braking balance during light brake applications. The valve does this by preventing application of the front disc brakes until 75 to 135 PSI has been built up in the hydraulic system.
This hydraulic pressure overcomes the tension of the shoe return springs on the rear drum brakes. Thus, the rear brake shoes move outward to contact the drums at the same time the pads in the front disc brakes contact the rotors."

Metering was not used on all disc/drum vehicles, and was removed on occasion when LOCO skid pad testing (very low tire to road friction like ice) produced early rear slide with light braking. It will not prevent "nose-diving" as some state, but can improve some vehicle's "pedal or driver feel" during very low pressure braking.

IMO, they should not be installed on any custom non-stock brake system, and that includes combo valves with fixed-value proportioning and pressure differential. If you want a dash brake warning light, use a separate stand alone pressure differential switch, but avoid the damm combo valves.

 

This was discussed on another forum recently.

The theory being that the weight of the fluid in the remote reservoir and supply line, bearing down on the MC reservoir will overcome the gravity drain-back pressure from the caliper(s) to the MC piston bore. If that works it is great as a #2 RPV will have some (if little) affect on residual fluid pressure holding the caliper piston out somewhat (pad drag).

Now if using GM METRIC LOW DRAG CALIPERS, one would think the RPV would be beneficial as a period step-bore MC is not available other than NOS or possibly a well sourced reman. That hopefully would prevent some instances of low pedal/delayed front application.

 

If you scroll down in the WILWOOD TECH ARTICLE I posted, you will find the pressure chart for an ADJ PROPORTIONING VALVE.

 

There was more than one car line released by 1965-66 FORD other than the MUSTANG.

A four wheel DISC BRAKE car does not need either a METERING or PPV IF the car is OEM as proper brake balance is designed for the particular car by engineers. The trouble begins when a rear disc swap is done and the rear system is incompatible with the front system, aka improper brake balance.

 

I'd still like to see a parts diagram for a T-Bird or Lincoln (the other two cars that got discs)....

I realize that making statements with no proof is in vogue these days, but hardly the way to make your case.

 

You need a somewhat more current tech source -

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SOURCE - https://www.freeasestudyguides.com/brake-metering-valve-test.html

Simply put, on a DISC/DRUM car, the front disc will engage long before the rear drum brakes are energized.

It takes a while for the fluid to move the pistons, have the shoes contact the drum (proper adjustment is critical) and for the shoes to self-energize after contacting the drum.

Now your mind is made up and I or no one else is going to change that. This info is for the novice that keeps reading and believing all of the OLD WIVES TALES that keep going on and on, much like the ENERGIZER BUNNY.

Of course, all of this is IMO ...

 

You forgot FORD and MERCURY.

Listen, you can either believe me or not. Your problem, not mine.

 

... sigh ...

BASIC PN 2B161 In The ILL -

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So, the fix for weak/worn front shocks is a new metering valve? Yes, I read the link. This explains why auto repair shops get bad names....

 

No, the biggest reason is untrained/non-certified techs using 30yr old technology sourcing the cheapest box store parts they can find and charging premium price for them. That and the usual customer rip-off tactics incl the ever so popular taillights driving out of sight warranty limit.

You still don't understand weight transfer do you?

 

I think both of you guys have put forth some interesting information that has made a lot of people think about what is/may be needed to make a brake system work. I think the discourse from two opposing view points is good. From a less technical point of view, I'd simply quote the old saying......"There is more than one way to skin a cat" or "there is often more than one way to do something" applies here. There are lots of variables when a hot rodder cobbles together a brake system. While this thread has discussed the purposes and effects of the various components related to the master cylinders function, the actual end result will also vary with the selection of the actual braking caliper/drum/shoe/pad used......as well as tire size/type and vehicle weight. Brake shoe material and the size of the drum probably will be different from what the original system had, and the original system may have been all drum brakes. On the front end, there are a myriad of choices for caliper designs and brake pads. It seems logical that the application of pressure to a shoe system is probably different than the pressure application to a caliper system, and may need some tailoring .......especially when generic components from various unrelated sources are used. While its possible to say that doing something one particular way will work correctly, I don't think its probable that saying all other ways won't work satisfactorily would be correct. Talk of tailoring the pressure by OEMs doesn't mean that their choice is the only correct choice. Maybe there was a range of pressure choices that were satisfactory and the one they picked was just one of several that would work. I don't think there is a definitive answer other than to assemble one that's similar to one that works well and then see what result you get. Many will be fine, and some will require some further troubleshooting. Lots of good info here, so Thanks guys...........

 

When a vehicle is designed/assembled, it must pass FED BRAKING STANDARDS TESTING before public release.

An OEM SYSTEM is engineered for the specific car. Modifying a car's braking system puts you out of and beyond engineered tech (college boys).

If one would sit down and write a comprehensive TECH ARTICLE, more than half here would say you're crazy.

And yes, differing friction materials (among many other factors) has a lot to do with the equation and final results (I love it when someone posts, "I found my pads at so and so rip-off store for ten dollars"). It is a scary world out there.

As an example, I had a 73 RANCHERO. If you spit out the window, the rear would come around. Simple fix, buy a new OEM PPV. WRONG ANSWER! The OEM PPV was for both the RANCHERO and TORINO SW.

Maybe basically the same car but vehicle weight bias is way off.

 

If the car did nose dive like that, it would indicate weak coil spring, not shocks.

Try this ...

Put coil over helper springs over the front shocks and lock 'em down. Or better yet, weld the stems so as not to move, or better yet again, just remove them entirely. Insert spacers between the coils on the springs so that they will not compress.

Apply a FULL PANIC STOP and report back here what the results were (once you had the front seat cover extracted from your anal cavity at urgent care), and I pray you had your lap belt on.

When rear self energizing drum brakes are applied (and if serviced/adjusted correctly) they will immediately drag the car down until the front discs are applied. This is called weight transfer. The front brakes perform approx 70-75% of vehicle braking. That sudden weight transfer will throw vehicle weight forward, and will compress the front springs.

That is the purpose of a FRONT METERING VALVE is to delay front brake application until the rear brakes (drums) begin to apply. The METERING VALVE will allow the front and rear brakes to apply simultaneously preventing front nose dive. This is called vehicle balanced braking.

There is no need for a PPV/METERING for OEM application (some exceptions) four wheel drum or disc if the system was designed properly. If one modifies a braking system in any fashion and causes an imbalance, it is going to require valving.

 

The METERING VALVE question is answered partially by the questionable quality KIT VENDORS pushing a GM style CHI-COM knockoff COMBINATION VALVE that includes the FRONT CIRCUIT METERING VALVE.

That answers that. It is done more for a legal reason I would assume rather than need/practicality considerations as the COMB VLV also includes the rear circuit PPV which is calibrated for who knows what certain vehicle.

IMO, if one uses this COMBINATION VALVE, the PDV section and the PPV section should be disabled and one plumbs in an ADJ VALVE behind it or source a true ADJ PPV.

FREE STANDING METERING VALVES are available as either take-offs (would require a rebuild kit), NOS if you can find them and aftermarket sourced who-knows-where the container ship sailed out of.

SOURCE - https://www.inlinetube.com/collections/brake-products#

Now listen, all of this is intended for the novice so hopefully he/she/it/mutant can hopefully understand what may/can happen when modifying brake systems. It is not meant for the OLD WIVES TALES GROUP. One's experiences and mileage can vary greatly.

HIGH HP STREET CAR EXPERIENCING EXTREME BRAKE FAILURE -

https://www.reddit.com/r/cars/comments/vups2d/1300hp_mercury_comet_crashes_due_to_brake_failure/