dean1484
dean1484 PowerDork
3/15/14 9:29 a.m.

My 94 has the PBR swap and I have been chasing brake balance in it since I have had it. As in I don't have much of any rear brake. I have changed the steel lines, the soft lines, the proportioning valve (to a 98GT one) new rear calipers and the master and it has made no difference. I am at a loss as to why I don't have any rear brakes with the PBR calipers from the later GT in my 94.

I am seriously wondering if the lines on the master are backwards. I have not tried swapping that around that yet. This summer I have to dig in to this again but if anyone had any suggestions it would be hugely helpful. I may actually swap back to the stock calipers. I got the car with the PBRs on it so I have never tried it with the stock calipers up front but driving the car the way it is in any kind of snow or even wet roads leads to front wheel lock up really easily.

With basically the entire brake system having been replaced I am now thinking it is a problem with the PBR's being not matched to the rest of the system. Or maybe the proporshining valve is plumbed backwards. OR???

I have thought of a manual proporshing (sp)valve but those reduce pressure to a given end of the car and this is a case that I want to either move pressure from the front to the rear or increase pressure in the rear while maintaining the same brake pressure in the front.

Anyone have any ideas on where I should go next?

The_Jed
The_Jed SuperDork
3/15/14 9:46 a.m.

Are the pads the same type (friction material)?

Are the rotors the same age/condition/size front to rear?

Maybe try running sans proportioning valve. This is the way I have my blue Mark VII plumbed and I like it.

dean1484
dean1484 PowerDork
3/15/14 10:08 a.m.

Pads are the same material. Rotors were replaced at the same time. What is a Sans Proportioning Valve?

pjbgravely
pjbgravely New Reader
3/15/14 5:12 p.m.

In reply to dean1484:

Sans means without. Isn't the an adjustable one you could use?

HappyAndy
HappyAndy SuperDork
3/15/14 6:05 p.m.

Do the cars that came with the PBR calipers use the same P/N master cyl and proportioning valve that your car has? That could easily be your problem if the are different, especially the M/C.

dean1484
dean1484 PowerDork
3/15/14 10:49 p.m.
HappyAndy wrote: Do the cars that came with the PBR calipers use the same P/N master cyl and proportioning valve that your car has? That could easily be your problem if the are different, especially the M/C.

I don't know. I will have to look up part numbers. I put the proportioning valve that belongs there (from a 98 GT) that matches the calipers. I will have to see if the master is different as well.

gofastbobby
gofastbobby Reader
3/15/14 10:53 p.m.

my 95 gt has a similar issue, all stock. I have come to believe that mustang brakes just need tuning.

JKleiner
JKleiner Reader
3/16/14 8:17 a.m.
dean1484 wrote:
HappyAndy wrote: Do the cars that came with the PBR calipers use the same P/N master cyl and proportioning valve that your car has? That could easily be your problem if the are different, especially the M/C.
I don't know. I will have to look up part numbers. I put the proportioning valve that belongs there (from a 98 GT) that matches the calipers. I will have to see if the master is different as well.

But that isn't the prop valve that belongs with two piston PBR calipers. The '98 brake system is identical to the '94; same single piston calipers front & rear, same proportioning valve and same master cylinder. The PBR calipers came in '99 along with different rear calipers, prop valves and master cylinders. Using '98 parts didn't change anything from what you had hence the imbalance.

Jeff

tr8todd
tr8todd HalfDork
3/16/14 8:39 a.m.

Start by doing the math on the piston sizes. You should know cross sectional area of the fronts and the rears. This will give you a ratio front to rear. Your proportioning valve is what gives you this balance. It also will have a ratio. You don't want to push too much fluid in any one direction or you will get lock up at one end and nothing at the other. Once one end sees enough fluid to do the job, the other end effectively stops getting more fluid. If there isn't enough fluid inside the piston bore, then the piston won't push the pads against the rotors. Factory brakes favor the fronts so idiots don't lock up the rears going around wet corners. If this is all too confusing, remove the stock valve and install an adjustable one. Get it to where all four lock up evenly , then dial it back a smidge until just the fronts lock in a panic stop. One other thing to consider is rotor run out. If you have even one rotor with excess run out, it will push the piston deeper into its bore, requiring more fluid to engage the pads against the rotor.

dean1484
dean1484 PowerDork
3/17/14 7:07 a.m.
JKleiner wrote:
dean1484 wrote:
HappyAndy wrote: Do the cars that came with the PBR calipers use the same P/N master cyl and proportioning valve that your car has? That could easily be your problem if the are different, especially the M/C.
I don't know. I will have to look up part numbers. I put the proportioning valve that belongs there (from a 98 GT) that matches the calipers. I will have to see if the master is different as well.
But that isn't the prop valve that belongs with two piston PBR calipers. The '98 brake system is identical to the '94; same single piston calipers front & rear, same proportioning valve and same master cylinder. The PBR calipers came in '99 along with different rear calipers, prop valves and master cylinders. Using '98 parts didn't change anything from what you had hence the imbalance. Jeff

Nope they are listed on 98's as well and there is a different prop valve for the 98GT as well (What I ordered) when I changed it. I just put new pads up front and got them for a 98 GT. That is how I know that the PBR's were on 98's .

I am interested in the fact that there are different rear calipers listed (I had not looked at that) and possibly a master difference. I think I should probably order a complete setup that matches and install that and see what I get.

I will also look in to the physical sizing of the components and see if that tells anything.

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