I have a 90 model. Brakes just didn't seem to be stopping as they should. I've replace the rotors, pads and all the hoses. I've flushed all the old fluid out with all new synthetic back in. The brake booster is good. Seems to have more pedal travel than it should and brakes just don't seem too good. Thought maybe the master cylinder was week. Any ideas? Thanks

 

Master cylinder? There could also be a problem with the anti skid unit not porting fluid properly, shuttle valve stuck? There also is a master cylinder mod where a different spring is substituted which changes the fluid pressure being applied to the front and rear brakes, don't remember where I read about the spring. I'm not sure synthetic brake fluid is approved for disk brakes, the high temperature of the brake assembly could be affecting the fluid.

 

From the maintenance manual: excessive brake petal effort - leaking vacuum system, restricted air passage in power head, damaged power head, damaged power head, restricted brake fluid passage. Also use only DOT 3 brake fluid, do not use DOT 5.

 

Vacuum is good, Used Dot 3 fluid.

 

I have an 84 and had the identical symptoms. To make a long story short, Once i knew the master and booster were good, I found a shop that had a brake pressure guage. This guage hooks in at the bleeder valve. Thye were able to verify i have plenty of pressure to the calipers. After that, I changed to HAWK HPS pads. THE PROBLEM WAS SOLVED ! !

 

Here is another thought. The manual says: excessive brake pedal travel - there could be air in the system. Some times air gets trapped in the system and you might have to bleed again. Sequence: right front, right rear, left rear, left front. Maybe manual bleeding will not dislodge air and you will have to pressure bleed.

 

Another possibility; the anti-lock modulator valve (this is where i think air could be trapped). The unit also could be bad. Let us know how you make out. Did you have excessive brake pedal travel before replacing brake components?

 

The pedal moves about an inch. It just seems the pressure is low as they just don't seem to stop as they should. Where is the anti-lock modulator valve and how can I check it? Thanks for all the advice.

 

It is located under the rear storage compartment behind the seats. When you start to drive you should hear a motor running sound (second or two) this is the valve testing itself, if a problem is detected the ABS light should come on. (there still could be a problem) I would try bleeding the brakes again, I think there is still air trapped in the system, maintenance has stated the modulator valve sometimes traps air. Trapped air will cause excessive brake pedal movement and a spongy brake feel. Your might have to take your car in for service. Please let us know the outcome.

 

If the entire system must be bled, the master cylinder prime pipe should always be done first. (from my 94 fsm) The M.C. prime pipe runs to the modulator valve in the compartment behind the seat. I pretty much did what you are doing except I let the M.C. bleed down and had to remove it to bench bleed it. The prime pipe bleeder at the modulator is a bit*h to get to, not sure if this is part of your problem but food for thought
larry

 

I have been going through this ever since I did my brake change. The brakes were never very good to begin with, and I suspect the fluid was at least a decade old if not original. I upgraded. To J55 calipers, 13 inch slotted rotors in front and stock size slotted in rear. Pads are street ceramic type. The pedal would go half way down and would need a second pump to build some pressure to stop, but even then it would not be enough to get the ABS going. They felt very long even after repumping to build some pressure. After much reading and as a last resort before going for a master cylinder swap, today I bled the brakes again, but I included the ABS unit in the bleeding process.
On the first pumping I heard a blob of air coming through, and as I looked back I saw the aerated fluid going into the catch can.
I pumped until the fluid was clean and bubble free, and moved on to the right front wheel, repeated the process until clean and bubble free and continued in order to right rear, left front and left rear wheel.
The left rear let out the most amount of air bubblel of all tires, almost comparable to the ABS unit.
After bleeding, I test drove it and I almost went through the windshield with the same braking effort I had been using; a firm pedal and single pumping as it should be.
I tried it on a longer stretch, and finally I was able to feel how the brakes are supposed to work in this car.
I used Valvoline Synth fluid DOT 3/4 .

 

Bled the brakes again. Checked the modulator. It doesn't appear to have a bleeder on it. The FSM says nothing about bleeding it either. Anyway, I bedded the brakes today. Most of the zinc has been removed from the braking surface. They appear to be getting better since. Anymore ideas, I'm listening. Thanks for all the help.

 

Upgraded to C5 Zo6 brakes on my 92 ZR-1 today, had the same issues, bled and bled still had a spunge pedal, so I bled the modulator and the pedal feels alot better, in the am I will re-bleed all four corners. I cant believe how much air was in that unit.

 

Apparently no bleeder on the 90 modulator. I looked at it and can't find anything on it in the FSM.

 

if you open the panel behind the drivers seat on the left side of the modulator is the bleeder valve, not sure about 1990 but that is where mine is located

 

OH, I opened the compartment. Looked all over the modulator. I didn't see a bleeder anywhere.

 

I have a 1990 and there is no bleed valve on the modulator, only thing I could think of is to loosen the brake lines going in to the modulator and try it that way. I also remember seeing a thread on activating the modulator as you bleed, I cant find the thread.

 

I watched a video today that said to bleed the modulator then turn the key to the on position wait 10-15 seconds, then repeat all the steps again. I don't know what the hell the deal is, I bleed and bled everything and still with the air, I decided to take the MC to oriley's, they send them out to be rebuilt, and I ordered rebuild kits for all four corners. I am frustrated..... the pedal is better but still not right and I don't know what I am doing wrong, I been bleeding breaks all my life with no issues like this. anyway anyone comes up with something let me know.

 

Re-bled all the calipers in the proper order. No air up front, but the rears both had air in them again. I've gone thru 1 1/2 quarts of brake fluid so far. One thing for sure, it's all pretty clean. The brakes seem to be better. Not sure if they're as good as they should be, but they seem to be much better.

 

Did you bleed them in the correct order? That's part of the key to it.