Brake prop valve or distributor block?

[Spetz]

I just got off the phone to a brake specialist and he gave me some info that I am not entirely sure is correct.

I have a brake distribution issue where only 9% of the braking is going to the rear.
My assumption/hope was that this was caused due to the rears having been converted from drum to disc brakes and all I needed was a brake prop valve from a car with the same brakes as upgraded to (the car is a 2001 Mitsubishi Mirage with Evo III brakes all round).

Anyway, the guy said that the OEM "brake prop" is not a brake proportioning valve at all, but it is a distribution block only and the BMC is what regulates proportion as it is a tandem twin piston design and hence the first piston works harder.

This seemed really incorrect to me. Can anyone confirm whether there is any truth to this at all?

 

[buck91]

I cannot confirm this but have heard similar about other vehicles before. I am curious how/where he got the 9% number from, though.

 

[Spetz]

The car went onto a brake roller test and I was given these figures.

The brake specialist did not test or see the car

 

[chrome]

Spetz,

I do believe the brake specialist may be correct. If I recall correctly, that vintage of Mitsubishi only has a distribution block, and not a proper proportioning valve. The pressure proportioning is done internally by the master cylinder.

Did you end up getting the larger MC? 15/16" or 1"? Along with the twin chamber vacuum booster? I would suggest the 1" MC for a nice firm pedal feel especially since you have upgraded the front calipers to twin pots.

If upgrading the MC does not resolve the low braking force on the rear, start looking at the rear pads too. The OEM rear pads seem to glaze pretty easily, maybe because of the type of friction material.

 

[Trav]

Unless the MC has 2 different size pistons I cant see it working as a proportioning unit. Generally vehicles with ABS and 4 wheel disc usually do not use a proportioning/combi (includes brake warning switch) valve but the ones with drums in the rear use one after the abs unit.
On non ABS cars the proportioning/combi valve will be right after the MC going to the rear drum brakes (4 wheel disc do not use one). If there is a valve still there going to the rears for the old drum brakes it needs to be eliminated.

Edit: I have seen different size pistons in one MC (the front smaller than the rear but its extremely rare and from the look of the Mitsubishi unit it is not.
Yours was disc/drum before being converted so there is a proportioning valve in play and still there unless it was eliminated regardless if it has ABS or not. ABS will only dictate it location on the vehicle.

 

[Spetz]

The BMC on it now is 7/8" and the one that I am replacing it with is 15/16"
I didn't want to go 1" as the car has no ABS and will be driven on the track so I was concerned of locking up too easily.

As for the booster the old one is a dual stage and much larger.
The new one (that has come with the 15/16" BMC) is smaller.

I cannot interchange them though as the bolt go on different angles between the two.


I am still unsure of what to do.
I have rebuilt all the calipers, changed the BMC and booster, so I have the option of:
1. Leave it as is and get it tested
2. Try to find a different prop valve and get it tested


I am looking at Mitsubishi parts numbers and the FTO has the same part number brake prop valve as my car (not 100% on this as I am looking at Japanese part numbers and I am not sure if the Mirage/Asti there had drums or not at the back)

 

[meep]

I'm not sure if this is relevant, but is the prop valve located in the back and configured as a "load sensing" valve?

I ask that because there is a misconception that the load sensing types actually do any proportioning - they do not. This seems to pop up in other drum-disc conversions. The load sense prop valve basically just limits fast in-rush of brake fluid, such as when someone slams on the brakes. If the application of the brakes is sudden, based on the amount of rear sag, it will block the flow to the rear. But if the pressure is applied gradually, fluid goes through unaffected. Thus, these actually don't proportion fluid pressure front to rear, they just avoid immediate lockup under quickly-applied brakes in a panic situation. Thus, folks who do drum/disc conversions in these applications can upset the front/rear bias unknowingly, with no "tinkering" with the load sense prop valve offering no help to compensate.

-m

 

[Trav]

Does it have a proportioning valve on it? If it does and it should have unless you removed it during the rear disc conversion pull it and replace with a distribution block. You can also install a variable proportioning valve is you need to.

 

[Silk]

Haven't had any old Mitsi's in lately, but a ''99 Legnum was in today, close in age to your Mirage. There was a valve body on the firewall, looks like bias split, and was more likely a load sensing valve as meep suggests. I would either gut the valve, or if that's not possible, then drill through.

 

[Spetz]

Yes the car has a what looks like a prop valve to me on the firewall. It is just the stock one.

It looks pretty much like this:
http://www.galantvr4.org/img.php?id=61558&width=400

Is that a brake prop valve?

I have sourced one from a non-ABS equipped FTO that has the same brake setup as my car and also changed the BMC, booster and rebuilt F&R calipers so I am really hoping this fixes the issue

 

[Trav]

Bin that thing and I bet its going to work as it should. Also I don't about the load sensing one in the back, I have a feeling they need to go also but try it.

Mitsubishi rear drum brake proportioning valve.

 

[Spetz]

The only reason I am not certain it's the brake prop valve is that the FTO and my car seem to share the same part number for it.

Anyway, either which way I will try and hope it does fix the issue

 

[Trav]

In that case remove the one(s) in the rear with an empty block. One or all of these things are causing the trouble not the MC. 4 wheel disc brakes don't use these things. When I did a rear conversion on a 69 Chevelle SS it was the booster, MC, dist block and 4 lines, that's it.