I have been searching the forum without any luck so here it goes;

I have a 92 coupe, my first corvette. About 2 weeks ago my brake booster went out, I replaced it and the new one cracked in a week. So I put a new one in this last weekend and it felt fine this morning, but on the way home my brake peddle started to get stiff and I noticed it felt like I had some drag. This correlated to a drop in my inst gas mileage from 18 to 12. And when I let of the gas peddle my car started to slow down like the brakes were on. I have no error codes or service lights.

Does anyone have an idea?

 

Sounds like the booster is getting excessive vacuum?

 

I didn't think you could get too much vacuum? could the abs cause this kind of problem? I might try bleeding the brakes three or four times and see if that does the trick. Might check the lines too.

 

Drive the car and then when you come to a stop somewhere feel the four rotors and see if one is hotter than the others. If so you probably have a caliper sticking. Just do not burn yourself.

 

I had the same issue when I did my brakes.
You probably still have air in the lines and close to the calipers.
You should bleed all calipers starting from the one farthest to the ABS unit.
Bleed order should be right rear, right front, left front and left rear.
I also bled the ABS unit and i had a bunch of air bubbles come out from the ABS bleed port.
I had to bleed twice (in the process, changed all the fluid too) and brakes have been great ever since.

 

Does your car have headers? I have heard of several people with headers that caused the brake booster to crack. If that's your situation then you need a heat shield between the headers and the brake booster.

 

hey cliff, I have read that too and I did buy some heat shield material. I don't have headers...yet; but I did notice that the OEM booster I took off was a lot more heavy duty then the replacement part. The plastic casing felt a little flimsy to me.

 

Have a few tips.
1 the brake booster diaphragm may have a tear or hole allowing the vacuum to put pressure onto the master cylinder without any pedal pressure being applied.

2 check the rod between the booster and master cylinder, although if it is too long it would not allow the brake pedal to go back enough for more fluid during bleeding the brakes.

3 There is a bolt on the side of the master cylinder that is supposed to be fitted when the pistons are fully depressed............... just a thaught but if you have not touched the master cylinder it should be fine..
I remember replacing my booster without disconnection the brake lines.

I recommend the metal brake booster, the plastic ones tend to break during reconditioning Or the brake repair shops refuse to deal with plastic boosters..

 

I will run all the lines and check the calipers this weekend and post what I find. Thanks everyone

 

The grabbing calipers to the point of slowing down the car is the same exact problem that I had. If at any point you disconnected any brake fluis lines, either at the calipers or at the Master Cylinder, I would say that your problem is most likely air in the lines.
When mine grabbed and slowed down severely, I managed to get home and took the front wheel off. They were grabbing so hard and where so hot that I could not attempt to remove them. When I hooked a clear line to the bleeder and opened it just a bit, the clear line immediately got covered inside with steam.
I waited for them all to cool down, inspected all the caliperes for no seal damage or leaks, and then proceeded to bleed them as described above. I took the time to do two full bleeding circuits until clear fluid bled out of all calipers and the ABS unit. They have been great now for several months and no further issues.

 

Boxcar here,

I’m interested in your bleeding order.

“You probably still have air in the lines and close to the calipers.
You should bleed all calipers starting from the one farthest to the ABS unit.
Bleed order should be right rear, right front, left front and left rear.
I also bled the ABS unit and i had a bunch of air bubbles come out from the ABS bleed port”.

Where does that information come from? According to Haines Manual (not the best manual acknowledged) you bleed furthest from the master cylinder. There is no mention of bleeding the ABS system. I have completely rebuilt all four calipers, new SS brake lines, new fluid (bleed them at least five time since rebuild), but I just don’t have the firm fast brake pedal I would like, it is still soft. I began to suspect the booster, but this intrigues me. I’m going to bleed in accordance with your instructions, and see if that does not give me the firm factory fresh pedal feel. This forum is great, I never even thought about the ABS system having air in it.

Cheers

 

So does my FSM say, furthest from Master cylinder, right rear, left rear, right front, left front.
I did consult with one of the head technicians at my GM dealer (guy is extremely knowledgeable in C4 and all corvettes from C3 on, and he said that when air has entered the system either by running the MC dry, replacing the MC, the FSM recommended action is to bleed the prime pipe at the ABS bleeding port, and then continue with normal routine bleeding. He said he had found that bleeding the ABS first and then going from farthest to closest to the ABS caliper and one last bleeding on the ABS, takes care of most air in the system. sometimes two rounds are required, but he usually gets good pedal feel with one bleeding rotation.
I tried it at home after several attempts at getting a good pedal (not having included the ABS before) and did as suggested. ABS first, then right rear, right front, left front and raight rear and final bubbles out of the ABS, and bingo. Even though the firts time the ABS bled clear, after bleeding all calipers and returning to the ABS I could see quite a few bubbles in the fluid coming out.
He also told me that common mistake when replacing calipers is not allowing enough bleeding time for new calipers to fill with fuid, and when they get got, the air heatd of the moisture creating steam that travels up, eventually getting to the ABS and master cylinder.
From what I have seen in most of my vehicles bought used is that they might have new pads and rotors to show a recent brake job, but the fluid is black and the calipers have normally pushed back in moving all the crap back into the lines.
I had been told before about the air in the ABS unit when doing the brakes in my jeep, but never thought about the location of it in the Vette as affecting the order of bleeding.
At any rate, even if you follow the order recommended in the FSM, be sure to check the part that shows bleeding of the prime pipe at the ABS. There most likely will be some air in there.

 

OK, here we go....
I to did some research and here is what I found. The source is from StopTech Brake Company: http://www.stoptech.com/technical-su...-brake-systems

Give it a read; it states that the ABS Valves need to be cycled in order to flush out old fluid and air. To this, you need a dealer service tool...or...get this, drive fast and slam on the brakes there by causing the ABS system to function thereby pulling new clean fluid in, then bleed the brakes again, that this may need to be done twice. Give it a read.

I looked at my ABS system box, and there is no "bleeder", I do not have a FSM (need one), so please take pic and point out this "prime pipe". I'm not cracking the ABS unless I'm 100% positive about procedure.

T/M on the information

 

Your 95 might be different than my 93.
All I did was open the compartment door in the hatch's dricer side, and the bleed port was right there looking at me.
There was no disassembly involved, only a quick bleed juat like you'd do with the callipers and reegular bleeders (open and close procedure and holding the brake pedal down) as opposed to bleeding with speed bleeders.
I do not have an FSM for 1995, so cant help you there.

 

According to this link, your 1995 got an updated Bosh ABS unit that has solenoids for each circuit, therefore less likely to trap air and can be bled normally.

http://www.aa1car.com/library/abs_bleeding.htm

 

Great link, and spot on. T/M

More research is needed. I really do want that factory fresh firm peddle.

 

There are times when a leaking caliper piston seal can across like a one way valve, sucking air when it retracts but "closing up" when it pushes. You would not necessarily see a leak if this is taking place. I would pull the calipers and examine the seals closely.

 

Getting ready for the Glen, and I'm making a list of brake checks, that just got added. Even though I'm pretty confident the piston seals are good. I rebuilt them last winter, lubed everything, then did a air leak test on the calipers. There were no air bubbles from the piston seals at 125 PSI.

I'm still suspect of the booster and MC.

 

Great work. Good luck chasing it.