sancho

This is in regards to the '89 IROC in the signature; PBR calipers in the rear...

The rear brakes simply will not lock. I've tested by putting the rear up on jackstands, putting the car in gear at idle, and then applying the brakes. The tires only moderately slow down, but will not lock even with the brakes floored. Applying the emergency brake will lock the rear, but only barely.

This is about the third post I've made on my rear brakes, and each time I've recieved some useful replies. I have installed new rotors, new pads, flushed the brake lines, bled the **** out lines, and even rebuilt the calipers with AC/delco kits. I've not really seen any improvement in braking power.

The second time I posted, people suggested to modify/replace my proportioning valve. While I'm sure there's some improvement to be had by doing that, I've got to think that the real problem lies somewhere else. I mean, if the rear tires can't even lock when they have no load at idle, how much use will they be to stop a ~3500 pound car going even 35 MPH? As far as I'm concerned, if they can't even stop the rear tires with no load, then they are virtually non-existant. Certainly even the mal-adjusted stock proportioning valve can't be that bad, can it?

I do remember that, after I rebuilt the calipers, the pistons were very tight when I tried to push them in. I can also remember that I tried to blow the pistons back out with compressed air, and that was difficult as well. I just chalked that up to them not being "broken in", but now I'm starting to wonder if that was a hint at my problems all along.

Do I need to be thinking about getting new calipers, or is it still suggested that replace my proportioning valve first?

Also, if I do need to replace my calipers, should I take the opportunity to upgrade to C5 brakes?

I'm just trying to get these brakes to be at <i>least</i> as good as the stock drum brakes on my '88 Camaro, which can lock up easily.

Thanks

AGood2.8

Let me state firmly that I promise (from what you have posted) that it is definately your proportioning (combo) valve that is shot.

First off- you live in Texas, a non corrosive state (pun intended)
thus your rebuilt caliper pistons are new and functional.

Secondly- you have replaced everything else.

Put in a new 1LE combo valve for your year car- here's the iffy part- the dividing line on the two availible 1LE combo valves is '89- some '89's had the smaller pitch threads, some '89's had the larger pitch threads.

I can definately tell you the 1LE combo valve for pre-to-early '89 cars with 1LE package is part # 14089496

sancho

Okay, that's what I needed to hear. Or, rather, I'd rather hear that than believe that I need new calipers (how on earth would you ever need new ones if you rebuild them?).

Regarding the thread size... I suppose I'll just have to pull the old one off and compare it to the dealer part. If it's the same, great; if not... hopefully I don't have to go down that road.

I'll be picking up a new porportioning valve tomorrow...

Thanks!

89 Iroc Z

I have been trying to figure out for about 6 mounts now which thread size my car has. Can anyone tell me a way I can find out?

AGood2.8

I just went out to look again at the one I put in a few months back (mine is the earlier 1LE valve I believe it was '85-to-early '89 cars)

From what I can visualy see (and remember) is that the two lines from the master cylinder into the top of the combo valve have the same size and thread pitch as the lower-larger line that feeds the rear brakes (bottom front line attached to valve)- The top and bottom rear lines that feed the front brakes off the valve are smaller lines on both early and late combo valves but the earlier one like mine has fine thread pitch on these line nuts that are very close in pitch to the three other large lines.

the later '89-92 1LE valve iI believe has more course threads on the one large on two small caliper feed lines coming off the combo valve. However the master cylinder lines are still fine pitch like the earlier unit.

Hope that helps.

Dean.

biff85ta

Have you tried changing the rubber hose that connects between the body and rear end? Maybe it has collapsed.

DannyT

It's funny that you bring this up because I was talking to two other Iroc owners last night and all three of us have the exact same problem. I first noticed it when I had my car dynoed and again when my car was smogged. Both times we were unable to stop the rear wheels from turning with the brake pedal fully engaged.
I haven't done much trouble shooting because I don't drive it that much but is the proportioning valve a common trouble for 3rd gen rear disc brakes?

AGood2.8

Biff,

Sancho stated in his first post that e had "bleed the **** out of them" so I am assuming from that statement that he is getting flow through that rear line. You will get flow from a bad combo valve also because the spring pressures adjust to front and rear pressure balance but will still flow even when pressure are not balanced. The fronts are getting adequate pressure so a bad valve will then allow flow to the rears when a line is opened. The assembly proportionately works against eachother to allow flow. From what I know about them- too much flow one way or the other and the safety light will appear.

sancho

biff85ta: It's funny you should mention that... I went out to three auto parts stores today chasing some rear brake lines for my car (not because I thought I really needed them, but just to cover my bases). When I finally came to an AutoZone that had them, the guy brought me two of the same thing. I thought that was odd because I know the two back lines look like different lengths, but I gave him the benefit of the doubt (my first mistake--never do that for an AutoZone employee ). When I jacked up the car to install my new "brake lines", I realized that what he gave me was two of exactly the part you're talking about. I inspected it while I was down there, and it looked just fine--definately not collapsed, anyway.

I have new brake lines on the front, at least--and I'd imagine that they'd be more likely to wear out since they must flex due to wheel steering. I think the lines are all in good condition since the brake pedal was hard and not spongy after I had the lines bled.

Chickenman35

Rear brake hoses can deteriorate from the inside. More common than realised. All sorts of crud can build up and restrict them. Good practise to replace them.

Advice on replacing Proportioning valve is valid. A bad proportioning valve can still pass fluid for bleeding brakes. But some crud stuck in a port will not allow enough pressure to be applied to the raer brakes.

It is quite common not to be able to lock the rear brakes on a Panic stop. The stock proportioning valve is Biased very much to the front. GM does this for the "Granny Factor "

What is NOT normal is that you cannot stop the rear wheels at idle when the car is on jackstands. That is grossly wrong.

You may have a defective Master Cylinder...however I go back to your statement when you rebuild your calipers.

Quote: "I do remember that, after I rebuilt the calipers, the pistons were very tight when I tried to push them in. I can also remember that I tried to blow the pistons back out with compressed air, and that was difficult as well. I just chalked that up to them not being "broken in", but now I'm starting to wonder if that was a hint at my problems all along. "

That does not sound right. After rebuilding, the pistons should be a nice smooth push back into the bores. They should easily " Pop" out with compressed air applied. Something not right there by the sounds of it. I'd invest first in some Reman rear Calipers. Try and get something quality like a Wagner, AC\Delco, Raybestos or Bendix unit. Stay away from no-name brands. Butuy new hoses all around...they're cheap and good insurance. Three hoses on back remember.

Rear pads also quite important. You don't need anything trick there. IE : Porterfields, Performance Friction. Hawk etc.

I autocross and hill climb my car. One of the best pads that I have found are the Bendix " Titanium " brand. Good hard pedal, excellant retardation, even when cold, and excellant modulation. FF rated ...they don't fade even under extreme conditions yet are easy on rotors. Can't say the same about the Raybestos pads. Poor retardation in general and a soft pedal to boot. How can pads cause a soft pedal? Raybestos uses an anti-squeel shim that compresses too much.

My recommendations.

1) Check Service manual and make sure that you didn't over travel Metering valve when you bled brakes. Don't think this is the problem though....but hey...it's free.

2) Quality new or Reman Rear Calipers. Those pistons sound tight to me.

3) Replace all of the hoses.

4) Replace Rear pads with Bendix Titanium.

If problem still persists.

5) New master Cylinder. Again get a brand name.

6) Last resort.... Ditch the stock proportioning valve and plumb in an adjustable WilWood unit.

Good Luck.

mystikkal_69

iTrader: (2)

agreed. the stock p-valve is pathetic. it's setup with just too much front bias. what brakes do u have up front? the non 1le?

"First Design"
non 1LE 1409497 w/M1.0 Threads Broadcast Code NB
1LE 1409496 w/M1.0 Threads Broadcast Code ND


"Second" Design used M1.5 Threads

Quasimo1

Sancho I was told by a reputable repair show that it's very common on our cars to not be able to lock up the rear disc brakes.

I drive a 90 Iroc camaro with a new master brake cylinder, and new rear calipers and still CAN NOT lock up (only slow them down) the rear wheels in drive when the car is on jackstands. However, I can lock up the rear wheels in neutral.

Just make sure that you use your e-brake all the time to keep the brakes adjusted properly and you should be fine.

sancho

All I can say is, if these rear disc brakes can't lock up, then GM would have been better off leaving drums on the rear. I think the whole reason that I have become concerned with the performance of the brakes on this car has been how they compare to my '88 Camaro SC, which has drums on the rear. I had that car on my driveway this weekend and I started testing out the parking brake--I would start backing backing out of the driveway and then pull on the parking brake. The car stopped instantaneously with just the parking brake. I then tried leaving the car in drive with the parking brake engaged, and I could go nearly a quarter-throttle before the car started moving. Plus, I can remember having that car up on jackstands and putting it into gear--I could rev up to 40MPH and hitting the power brakes would stop the tires cold.

Granted, the '88 Camaro has less than half of the engine of my '89 IROC, but the drum brakes have absolutely no problem stopping the whole vehicle in an instant. And I can't even remember the last time I changed the shoes on that car--it's been more than two years.

Anyway, I still plan to change out the proportioning valve...

Chickenman35: I think you're on to something with the caliper issues--the pistons were definately tight. If the proporting valve doesn't help, and new brake lines don't help, then calipers are next on my list.

Thanks again for the replies

Chickenman35

Not locking up in a Panic Stop is entirely different from not locking up the wheels while in drive on jackstands. Yikes!!!

While the stock proportioning valve is heavily biased towards the front it is not THAT bad. Something still not right with your system Quasi. I'd dig a bit deeper. Better yet...ditch the POS stock Proportioning valve and put in a proper adjustable Racing unit. A world of difference. Cheaper than a new GM unit as well.

AGood2.8

Its not cheaper to go to an adjustable proportioning valve- Most people do not have access or proper ability to flare new hard lines for the install- thus custom work needs to be done.

mystikkal_69

iTrader: (2)

my e-brake was the cause of my poor brake performance. they would not release and stayed in the applied position. manually adjusting them to be looser and not using the ebrake has given me fully functional rear brakes. make absolutely sure that you have no leaks of any kind. my brakes didn't feel the same this week and have discovered a slow leak. now repaired + bled everything seems fine again. believe me sancho i know how you feel. i was gonna go back to drums before i tried my last resort efforts. i replaced everything on the hydralic side of the system. what have you NOT replaced on your brake system?

Chickenman35

My flaring kit cost me $19.95. Readily available at Home Depot and Automotive Jobbers. WilWood's **** type proportioning valve is $39.95 from Summit. That's pretty cheap. A brass tee and some line fittings are less than $10. Far, far cheaper than a Factory valve. Don't think you can get the proportioning valves from any one other than GM.

sancho

Chickenman35: Would you recommend that I use a setup similar to yours as opposed to swapping in a GM replacement part?

I'm sure you're probably a bit biased, which is fine. All I really want to do is get the best braking power that I can for a daily-driver (with my existing calipers/rotors). From what I've heard, the best way to achieve that is to get the back tires to lock just after the fronts lock up. Can I get that with a GM replacement part--i.e. the 1LE replacement?

I can't think of why I would ever want to adjust anywhere outside of that...

TomP

A few weeks ago, I should've taken a picture of what happened with my dad's car. His front left brake hose blew out. That's a hell of enough reason for anyone to replace their brake hoses, never mind all the crap that's probably built up inside. We all should've changed our brake hoses (including front!!) at least once by now. The newest any of our cars can be is 11 years old.

They're only about $18 each. Damn good insurance against death, in my opinion!

Chickenman35

To get the adjustment you want you need an adjustable unit. The factory 1LE proportioning valve will not help. In fact I believe that the 1LE cars and the J65 cars use the same proportioning valves. (Edit: Confirmed. Just got off the phone with my Local GM dealership...whom I trust. J65 and 1LE proportioning valves are the same. Several revision were made, but this only pertained to line fitting size. Valves are identicle. )

The 1LE cars in stock form actually have even MORE front Bias ( Due to the larger front rotors and pads ) that the J65 cars.

I Autocross and run a lot of rear brake bias to help the car turn in. I can actually adjust the brakes to the point that the rears lock before the front!!! Not recommended BTW LOL.

Braking performance is dramatically increased. My car currently runs 10.5" rotors up front and the 1LE setup on the rear ( It's a heavily, heavily modified 1986 SC . ). Car will amazingly out brake 1LE cars...even 1LE cars running Track days at our local Road Course. Some super trick cooling options needed for that!!! ....including water sprayers, synthetic Motul fluid, Hawk Blues....but it can be done. Front pads and rotors were toast at the end of the day ....but they never faded.

Currently I am building a 12" x 1.625 rotor setup with Wilwood Superlites for track days. Rotors were spares from my bud's GT1 Corvette. $25 each

The brake Bias on 1LE cars is even worse than the J65 ( 10.5"frt\11.8"rear option ), as mentioned. With an adjustable proportioning valve you can vary the Bias more than enough with either the 1LE package or the J65 package to suit any condition.

BTW, Third Gens aren't the only cars afflicted with this problem. I had an '82 RX7 with four wheel brakes that had the same front Bias issues. Understeered like a pig at Autocrosses. Swapping in an adj proportioning valve and adding some rear Bias dramatically increased the braking ability and eliminated the turn-in understeer.

icecold

i've got a 90 firebird with the four wheel disc option.what a joke it is. it has exibited the same brake problem as you described since it was new.(will not stop the rear wheels at idle in the air) i currently still have at least 10 warrenty repair slips from when the car was still under warrenty and they are from different shops.i've swapped prop valves to no avail so i did the mod and let me tell you IT WORKS.
i dont think the factory valve is going bad i think that it was always junk. but with the fronts responsible for most of the braking most people dont notice.
i dident notice for 2 years unitl one tome i took the car out in the snow and couldent stop without putting the car in neutral.
the e-brake has more power,that just isent right

pete