I think I might have a master cylinder problem. My vettes been sitting for over 3 weeks. Now that I finally get it started and driving again....I think I'm starting to notice a light pedal....almost had to scoop my foot under pedal or tap it to bring it up. Assuming I can get the part(master cylinder).......Is this something that isnt extremely difficult to do with some friends' help on a '90 or is this something that's better to be done through a mechanic? Also, how long would this take hour-wise and/or does anyone have a list of instructions to change this out? I see its pretty easy on some cars, but possibly hell on others.

 

If your are sure that you don't just need a good bleed/flush, a brake master cylinder change, from start to finish, thirty to sixty minutes. You must bleed the master cylinder and the brakes at each wheel.

 

30 to sixty minutes? Thats it? I know you have to bleed them all as new air is introduced into the system from what I was reading about the change on other cars. But is this something a mechanic has to do for some reason or could I just do this with some help of friends and a Chiltons assuming I can get the part? And is there anything special I need for this? TiA

 

You can do it yourself. You'll need another person to help with the bleeding once the new MC is installed, unless you buy the special caliper bleed screws which allow one-person bleeding.

Mounting the new MC and bleeding the system is pretty straight forward, just be sure to follow the directions in the manual on which to bleed first, second, etc. My 86 FSM called for a different sequence than I'd been use to; GM must have had a reason for altering it.

Jake

 

Your 90 has ABS, right? There is a bleed screw on the ABS that has to be done first on my 92. Some of the later cars do a diag sequence on start I think, on mine it's just a manual bleed starting at the ABS. Your FSM will tell you how to get all the air out.

 

I just changed the master cylinder on my 90 a couple of months ago. It takes longer to bleed the brakes than to change the cylinder. I didn't have to bleed the ABS unit.

 

The issue of bleeding the ABS has come up on this Forum several times over the years. From what I've read in the FSMs (I have for both my 86 and my 96, both of which have ABS), and from what others have posted, there is no special bleeding of the ABS required when the Master Cylinder is changed.

The ABS secondary system is normally closed whenever the ABS is NOT functioning, so simply removing and replacing the Master Cylinder will not allow any air to enter that system.

If, however, the brake pressure modulator valve has been replaced or there is a reason to suspect air has somehow entered the ABS system, the ABS has to be cycled to purge the air. This calls for a special tool to cycle the ABS pump to cause the air to be forced to the brake calipers. It's at the calipers that the air is bleed out.

To make bleeding go quicker, it's recommended to first bench bleed the new Master Cylinder. All you need to do is lock the new MC in a vise; open the secondary port, use something (I use a blunt tip philips screwdriver) to depress the piston and while holding the piston "IN", tighten the secondary port.

You can also bleed the new MC with it installed by loosening the front most line, depressing the brake pedal, and while still holding the brake pedal to the floor, tighten the line. Wait several seconds and do it again. Keep doing that until all air is removed.

I then move to the brake calipers and bleed them according to what the FSM recommends, RR, LR, RF, LF. Since I don't have bleeder screws to allow me to do the job without the help of a pedal pumper, I make sure he/she pumps the brake pedal slowly.

Jake

 

For me the hardest part was getting the old resevoirs installed onto the new MC, what a PITA to slide them down over the new seals...

Also, you are told to bench-bleed the MC before installing onto the car.

Then bleeding at the wheels is greatly reduced, though it's a great opportunity to flush the entire brake system of old fluid, recommended.

 

I fudged your question.....It really didn't take that long. I think it was under 30 minutes. I didn't bench bleed the master cylinder. I put it on (two nuts) and two fuel lines and poped on the electrical connection on, filled up with Valvoline synthetic, and did the bleed on each wheel. If you install "Speed Bleeders", you can do it real fast and with the wheels on.

 

Try this:

pop one front and one rear caliper off (2-bolts) after putting in the new bench bled m/c. Compress the piston in by hand about 1/4", or use a c-clamp if it's too difficult.

This will force any air introducted up out of the system, no bleeding necessary.