Rear drum to disc conversion question, Do I need to do anything with the master cylinder? Options

[GriffGirl]

Someone told me he thought I will need to do something with/replace my master cylinder when I convert my rear drum brakes to disc brakes; something about the proportioning valve.

Here's what he said:

In regard to your brake swap, I could be wrong, but I'm almost positive that you'll need a different proportioning valve...When you do it, though, you should see about swapping the proportioning valve and
master cylinder (if it's different on the GTs)... Drum brake wheel cylinders and disc brake calipers use different amounts of fluid (typically) and if you swap from the former to the latter, your rear discs brakes won't be doing as much work as they should.

This is all greek to me. Help? Is this something I should look further in to?

[njccmd2002]

ok pretty much what he tried to say.

you apply the brakes, that is 100% force.

the master cylinder spreads the force to a 60% to the front and 40 % on the back ( hypothetical )

so by changing the configuration too much or too less % will get to the back.

IDK, thats the way i was explained, i coudl be rong. maybe it does not even matter. you will need to get the braking specs of the drums and calipers.

Maybe it does not matter, hope it helps.

[alltracman78]

That's pretty much how it works.

The P valve only allows a certain bit of pressure to the rears.
Too much pressure and the rear wheels will lock up.
Drum brakes get less pressure than disc brakes, and also get the pressure first [drums have to move further to start working] so ideally you want to replace the P valve when you add the discs.

I'm not sure if the master cylinder is different as well, or the same.
I do know on the 5 th gens the ABS cars have a slightly larger MC, so it's a bit of an upgrade for those that don't have it.

[GriffGirl]

Okay, so just to be perfectly clear. And btw I probably should've mentioned - I don't have ABS.

I should replace the proportioning valve when I do the conversion.
What should I replace it with? Do I try to find a proportioning valve that is intended for the GT? Or is it something that is self-adjusting when replaced, effectively replacing it with the same thing that I have now, but new? Also, is this a part that is easy to find?

I do not need to replace the MC but it wouldn't hurt to do so.
My understanding is correct here, right? So if I were to replace the MC, would it be with a GT part or the ST part? And were I to replace the MC, is the P Valve automatically a part of that? I have NO idea what a proportioning valve is anyway. But I'm about to find out it seems. (Cool!)

[GriffGirl]

Okay, I found the proportioning valve on partsamerica.com. And the MC of course.

Question: do I need them for the ST, or the GT since the rears will be converted? And can I save myself approximately $200 and get them used like from someone off here?

[DEATH]

They [PV and MC] are the same. The only difference is the regular and 4 channel ABS setups. Just going from nonABS ST to nonABS GT there is no difference.
Griffy will be along to confirm this shortly.

This post has been edited by DEATH: May 6, 2008 - 1:45 PM

[GriffGirl]

Yep, here I am. Part numbers for ST and GT P-Valve and MCs are the same. So I guess I've ultimately answered my own question here. But not without the love and support of my fellow 6gcers. Awwwwwwwww..........

[alltracman78]

Good to know.

What I meant by the MC was that there might be an upgrade available, not that it was necessary to change it.

A larger one will give you more braking force, but depending on the setup isn't always the best idea.

[phantom]

i replace my drums for disks and i think it brakes good enough. the e-brake works a thousand times much better than the drums did. but doing that change isnt "really" necesary, right.

[alltracman78]

They [PV and MC] are the same. The only difference is the regular and 4 channel ABS setups. Just going from nonABS ST to nonABS GT there is no difference.
Griffy will be along to confirm this shortly.

So because this didn't make sense to me, I don't trust most aftermarket stuff, I was curious [and work was slow today], I checked this.


There are three different P valves.
1 for all GTs, 1 for the STs without ABS, and 1 for the STs with ABS.
I couldn't check without using VINs, and I'm not as familiar with the 6th gens as I am with the 5th gens, but I'm assuming all GTs have 4 wheel disc, and all STs have rear drum?
If that's the case, they are different depending on rear disc/drum.

If that is true, I would suggest changing the proportioning valve along with the rear brake components.
If you don't it's not the end of the world; I'm currrently using a FWD P valve on my Alltrac, and it brakes just fine. Also, I'm sure others that have done the swap are using the valve with no problems.
But, it will most likely improve your braking quality a bit if you do, and make your rear wheels less likely to lock up.

I didn't check master cylinders. Maybe one of these days I'll dig through it.

[GriffGirl]

I'm researching this part - on 1stToyotaParts.com, there is what they call a Pressure Metering Valve. Is this the same thing? The options for this part for a GT are for GTs w/ABS. There is no w/o ABS option. Am I to assume all GTs came w/ABS? If so, how if at all does this affect me? (No ABS)

[alltracman78]

For something like this you can't use the online stuff for 1sttoyotaparts, it's not even close to complete.

Part #s for the proportioning valve.
47150-20370 ST W/ABS
47150-20280 ST W/out ABS
47150-20240 GT

Originally there were 5 different proportioning valves, but Toyota superseded several part #s.

The master cylinder is also different, but only for ABS or non ABS.
I assume the ABS cylinder is larger, like it is on the 5th gens.

47201-2B090 W/out ABS
47201-2B050 W/ABS

These are for a 94 Celica specifically. Dunno for other years.

[GriffGirl]

Bowchikawowow, my stuff came today!!!


Too bad it'll probably be another two weeks or so until I get the conversion done.

[D-Man]

YAY! for rear disks!!

[GriffGirl]

Question:

Is this the proportioning valve?

[alltracman78]

Nope.
That's just a side to side [front IIRC] splitter.

[playr158]

proportioning vavle



how it ties into the system + the Front splitter "T"



This post has been edited by playr158: Jun 9, 2008 - 10:56 AM

[alltracman78]

You don't have ABS.

IIRC her "splitter" is for an ABS equipped car.

Griff, that's one thing you need to pay attention to when you try to pick one up.
If you don't have ABS you can't use an ABS prop valve.

[Batman722]

I've done the drum to disk swap on 2 cars and neither needed a different valve.
The GT and ST master cylinder are the same.
All of your ST stuff will work fine except you need (GT) the backing plate w/e brake assembly, rear disks/pads/calipers, the rubber lines, and the cables to the e brake handle (all GT stuff).

[GriffGirl]

^I have all of that stuff you listed. It was the proportioning valve that came into question; it's my understanding that it is not something that is required for the conversion, but if swapped, will increase braking efficiency. I just wanna make sure I'm thorough and cover the details. I'm kinda OCD like that.

What may happen is if I don't find a p-valve by the time I do the swap, I'll do the swap w/out it, and swap out the p-valve at another time when I do find it. If it goes that way, it'll be interesting to see if there is any difference in braking efficiency.