Rear disk bleed fail - proportion valve removal

[3X00-Modified]

I think it all depends on what caliper you are using as well... Not all are created equal as far as how much flow is necessary to get you certain pressure.

[Barry]

Yeah theres a ton of things that affect it. But that also means theres some options out there to make the bias more agressive. A bigger piston rear caliper with a bigger rotor will grab a little harder with the same line pressure. Problem is that it takes more fluid to do so.

[woody90gtz]

You do want less brake in rear than front though, because under hard braking weight is transferred from the rear to front and the rears lock up easy. 1/2 is probably not bad.

[3X00-Modified]

When we setup the race car we always shoot for 60/40 braking bias starting point, and sometimes move to more front... so I'll agree with Eric on that statement.

[wicked-irocz]

well, our cars are in-between 70 and 80% front braking. If you have a good working Abs system it will help regulate this very well, even with a swap to rear discs. Your all very right on the comparison of volume as thats what matters and is likely why Weba's and Jon's work fine. They have larger front brake pistons requiring more travel and therefore more fluid makes its way to the rear. Simple enough.

[scd88ga]

well, our cars are in-between 70 and 80% front braking. If you have a good working Abs system it will help regulate this very well, even with a swap to rear discs. Your all very right on the comparison of volume as thats what matters and is likely why Weba's and Jon's work fine. They have larger front brake pistons requiring more travel and therefore more fluid makes its way to the rear. Simple enough.

So are you basically saying that converting to rear disc without first upgrading your fronts is a waste per say?

[3X00-Modified]

I've said that from the beginning since the fronts are very useless if you start to increase the power of your car... But many disagree with me.

[yellow3800]

I think boner had the 4 wheel disk figured out to the best I've seen. F-body brakes up front, rear disk in back, and your option to retain the 5x100 or change to 5x115 bearings. now I get the idea to PM him, lol.

the update is that I verified my pontiac 6000 master cyl part # [to avoid confusion as to what of 2 master cyls I have is actually on the car] because they are identical with exception of the insides of the main body tube only because I didn't dissect them both yet. I re-blocked the caps of the proportion valves by putting the springs on the bottom of the chambers so that the black taper seal does not plunge down and stick if I 'm trying to bleed the left rear stopping the fluid. Now that I know this is not happening, I still am not getting much fluid at the left rear. not enough to bleed anyway. of course, right front is bled first, and is okay. I've tried bleeding also by caliper-to-resevoir... no change.

By the way, the front-left and right rear balance out perfectly- on a wet road, I can nail the brakes, and the rears to not lock up at all, I actually dial it in [one variable proportion valve with knob $40ea] so that the fronts are the first to lock, though just barely. The wear will reflect the same as front getting bias= fronts wear first and brake dust is on all 4 wheels, heavier in the front as we'd all naturally think it would look like.

Could I have my lines messed up? and I need to verify this for pontiac 6000, though I believe since they use the same fittings, one per wheel at the mstr, cyl, that they are the same. I might be wrong. both masters are designed the same with the same holes in the same places, same size - so I hooked them up the same. My 89gt as a reference shows as looking at the master cyl from front right wheel:

Top left= front left
Bot left= right rear
Top right= front right
bot right= left rear

the two big outlet holes, one per chamber feed the front brakes. this makes sense because they need more authority. the bottom holes feed the rear for the same reason, only less. they are crossed because if one line fails, its a failsafe that both front don't fail at once. this way, at least one front brake works in event of a ruptured line, and this still makes sense.

What I don't understand, is the the front chamber of the cyl feeds the right rear brake, and this is fine on my car with the guts of the proportion chambers [or failsafes] removed. why doesn't the rear chamber? the rear chamber gets pressure [first] because its closer to the beginning of the master cylinder input. If I am wrong in assuming that the original proportion valves are the innards of the chambers of the mstr cyl, then my front left and right rear should not work. They do, and I figure so should the left rear.

I'm next tempted to swap my lines, so that the problem line [cannot bleed left rear] will move to the front right, and the healthy front right will go to the left rear. Maybe that way, the pressures will change, and allow a good volume of fluid to get to the left rear caliper, and the front right at that point, will flow just because. Doing this means that I'm not matching production assignments.

I should also verify that pontaic 6000 mstr cyl part # to be rear disk because years ago, the guy behind the counter said it was when selling it to me after looking up rear pads for the car and matching it. if it wasn't, and I had too much rear bias, I'd never know anyway.

Lemonjuice<---headaches all the time and nothing breaks twice.

[3X00-Modified]

I think boner had the 4 wheel disk figured out to the best I've seen. F-body brakes up front, rear disk in back, and your option to retain the 5x100 or change to 5x115 bearings. now I get the idea to PM him, lol.

When did he have it figured out? and if he did it was only on paper, he never did it...

I can run F-body's up front and I have the Neon's in the rear, but I feel with the N-body's up front and the Neon's in the rear that the factory bias for my MC works out good enough, I don't have rear lockup issues and my fronts would probably be about 70-80% braking, and since thats not far off from stock I don't see the issues with my setup...

Now for you it's a different story... it sounds like you got a Disc/Disc MC and tried to compensate for the change but then may have over done that and now your getting way too much rear bias.

[88_GTU]

Just a quick question related to the topic, what is the usual operating brake psi for the beretta? Call me curious.
J

[Kris GTwhat?]

i have the daytona 11" rear disc and the f body 12" fronts, and havent had any issues. yes, there is quite a bit less clamping force on the rear wheels, but look at most rear disc cars. you can go 60-70-80k miles before even having to worry about the pads due to less use. when my breaks are on, i cant move the rear wheels when torquing the lugs while its in the air, so i know theres descent clamping force. i think you guys just worry too much. lol

[yellow3800]

Moderator, feel free to delete the post.

Aside from my original LEFT REAR failing to bleed, which is still unsolved, I compared part #s today and found out that the pontiac 6000 master cylinder advance sold me actually is the same part # stamping on the bottom as the Beretta one. rockauto parts confirms this to be wrong. I guess this is the danger when buying parts for seven cars at once.

So after maximizing my proportion valve, I find out that the rears [or just right rear in my case] will still lock up first.

THAT EXPLAINS WHY THEY ARE IDENTICAL - port size, chambers, etc. ...ugh.

I was so happy to get the part I just put it on. I did that with a flywheel once... um, I changed the bracket that the slave cylinder pushed on to bypass my minimum thickness mistake! its still truckin 60kmiles later strong!

I hope it will magically bleed when I put the pontiac 6k master on. they must have lost it in an exchange.

andy

[DanteGTZ]

Has anyone come up with any new information on rear disc pressure issue?

[woody90gtz]

I have a brake pressure gauge, but I forgot to see if it has an adapter for the Beretta and/or Neon bleeder screw. I'll update when I find out.

[Burning_Beard]

Aside from my original LEFT REAR failing to bleed, which is still unsolved

I have the same problem with my left rear, but I'm completely stock. I went through 4 bottles before I rage quit. I just figured there's air in my master. I haven't decided whether to do a bench bleed or get a new master for the hell of it.