Is the 87-93 master cylinder a direct bolt on for an 86 mustang gt?

stang-geoff · Apr 23, 2010

I'm just wondering. I'm taking my car to my school's shop. I've been having serious braking problems and if we diagnose the problem to be the master cylinder I'd like to buy an 87-93 master cylinder since my shop uses a power bleeder that only works with the modern nylon/plastic reservoirs.

RatStang · Apr 23, 2010

IIRC, the master cylinder for the 87+ goes into a 3 line proportioining valve while the 86 goes into a 2 line proportioning valve. You would need a 3 line to 2 line adapter.

That said... it's not difficult to bench bleed a master cylinder at all. Mount the MC in a vice. Run some vac line from the outlet ports back into the resevoir. Use a 3/8 rachet extension to operate the master cylinder a couple dozen times to get the air out and voila! No special bleeder required.

stang-geoff · Apr 23, 2010

cool, is there a guide that'll show me how to do the adaptation? I'm assuming two of the lines are going to adapt into a single line and the other line is going to be left alone. Also, what's the third line used for?

stang-geoff · Apr 23, 2010

I'm looking at a cobra master cylinder now since it's 2 port. Will the 2 port cobra master cylinder be a direct fitting?

One more thing, will these master cylinders be a direct bolt on to my factory brake booster?

RatStang · Apr 23, 2010

Once again, I'm working from memory and am not the expert on this but I think that you're right about the Cobra MC.

The problem with that MC though, is that I don't think it has the rear resevoir capacity to work with your drum brakes. You'd have to convert the rears to disk to run that MC.

I'll say it again though... It's EASY to bench bleed the OEM 86 MC. You don't need anything special to do it.

Hopefully 5L5 will stop in here and either confirm or deny. He's pretty knowledgable about the various brake parts and combos. What works with what etc...

Send him a PM with a link to this thread.... Mustang5L5

stang-geoff · Apr 23, 2010

I'm sure it is easy but I would still like to have the plastic/nylon reservoir for bleeding my entire system instead of going through the hassle of getting someone to help me do it manually. That's a good point about the cobra mc. I didn't think of that.

I wonder if any 87-93 GT drivers converted to the cobra mc with rear drums still installed.

RatStang · Apr 23, 2010

stang-geoff said:
I wonder if any 87-93 GT drivers converted to the cobra mc with rear drums still installed.

I don't think so. That MC just doesn't have the capacity to drive the slave cylinders for rear drum.

One option might be to convert the rears to the SVO drum setup. That way, you'd still have the 4 lug but with disk brakes.

Mustang5L5 · Apr 23, 2010

You can't use the Cobra MC. It's bore dia is too large for your brakes. Your braking effort will increase by a lot.

Why not stick with a 85-86 MC that is stock to your car? There is no benefit to changing MC's. Nothing will improve other than the amount of work you put into adapting fittings and such.

You can easily bleed the brakes the conventional way and get a good pedal. I bleed my brakes all the time ALONE with the traditional bleeder screw and bottle method. I get rock solid pedals each time. It is much less work than adapting a newer MC into the car.

BUT, if you really want to do it, you'll need the 87-93 GT MC as the cobra one won't work. A 87-93 prop valve would make things easier as well as the lines between the PV and the MC. Then, you'll need to make a new brake line from the driver's front wheel to the 3rd port in the MC, and then come up with adapter fittings for the 2 remaining lines to join to the bottom of the 87+ PV into your car. Like i said...easier to just get a bottle and clear hose and bleed the old fashioned way

stang-geoff · Apr 23, 2010

I'm a bit skeptical about the reservoir on the all wheel disc MC. Since an MC contains extra brake fluid to compensate over time as the pads wear down. I have trouble thinking the cylinders in the drums are going to consume all the reserve fluid under braking. This is all speculation though. I can be entirely wrong.

Well, I didn't get a chance to look at my car today though, so I will have some time over the weekend to decide and who knows it might not be my mc that's the problem. I doubt it though.

Mustang5L5 · Apr 23, 2010

What exactly are your braking problems?

stang-geoff · Apr 24, 2010

The problem I'm having is my pedal travels halfway down before my brakes begin to engage, My brake light is always on and I've attempted to bleed out the rear brakes and there was hardly any flow with the pedal pushed down with full force. I originally thought the bleeder screws were just clogged and I just needed to adjust my rear brakes, but it sounds more likely that it would be the master cylinder. It would explain why I can't bleed the back brakes and why the pedal is traveling so far down before the brakes engage. If i can just plug the outlet ports on the MC I can verify whether it's the MC or not. I think my school shop may have something to plug the MC.

I wonder if I could find some bolts with the right threads and possibly plug the ports with a couple bolts with some thread sealer. Than I could do this at home.

Mustang5L5 · Apr 24, 2010

Well, the rear drums are difficult to bleed if there is air in the system. You need to bleed the fronts very well before going to the rears. I know it's not the proper procedure but I found that you need the fronts bled well first before the rears will flow.

The brake light could be a faulty e brake switch as well. Keep that in mind.

You are right that it could be the mc, but I would also suspect air in the mc or lines as well.

I may have the fitting sizes on my laptop so let me boot that up and look.

stang-geoff · Apr 24, 2010

I already did the fronts and the brake light will occasionally flicker off as I'm depressing the brake pedal. I could go quickly unplug the switch to the pressure differential valve to see if it's being caused by my MC or something wrong with the valve/switch.

Mustang5L5 · Apr 24, 2010

The fitting sizes are 9/16-18 and 1/2-20.

The 87+ Mustang went to metric, so to answer your main question, it would require reflaring different fitting sizes on and running a new brake line to 1 wheel to accomplish. You can't use the cobra MC as it's too large in bore diameter and will create a hard pedal

stang-geoff · Apr 27, 2010

Well I found the problem on Monday and fixed it today. The car had a rusty, seized wheel cylinder that was allowing air in and the shoes were contaminated with brake fluid. The other side didn't look to good either and my self adjusters were in poor shape, so $100 later I R&R the entire rear brakes. It's working well now. At a $100 I kinda wished I just saved up and bought one of the rear disc adapter kits. there's an an95 at my local yard with the rear intact.

Oh, and I still have my brake indicator light on at all times. I don't know if my master has a small leak in one of the piston cups or the sensor itself is bad. I've unplugged the sensor on the valve body and the light turns off

Is the 87-93 master cylinder a direct bolt on for an 86 mustang gt?

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