I am replacing the master cylinder on a 1991 Aero 4.0 ext van.
I installed the old reservoir on the rebuilt master cylinder using the new reservoir seals that came with the rebuilt master cylinder. I then installed the rebuilt master cylinder, bled it using the pedal, and then pressure bled the entire system to remove the old brake fluid. Up to this point there were no leaks.

After I removed the pressure bleeder the new master cylinder leaked from both the bottom of the reservoir and from the pressure differential switch when I had someone press the brake pedal. Tightening the pressure differential switch did not help.

Is this a defective unit or did I miss an important step?

 

Did you lubricate the seals before you put it together? If the seals are not lubed, they may not go together properly.

 

I lubricated the two reservoir seals with clean brake fluid before I installed it on the new master cylinder. It would be interesting (if it wasn't so frustrating) to figure out why the reservoir seals and dp switch seals did not leak when the reservoir was pressurized and did leak when it wasn't .

 

I would go back to where you bought the part and get new seals. I have had problems like this before - took the defective gasket or seal back in and the person at the counter just gave me new ones from another kit. Either that or you can just bring the whole part back in. That would mean re-bleeding the system

 

Is it leaking from the brake pipes or flare joints or from the seals ? Have you got damage or missing gasket at the pressure switch? I remember having a bit of a problem with the tube nuts when replacing the brake pipes. Getting them cross threaded was a possibility. However everything was O.K in my case. I also took quite some time getting the air out of the system. But it was all worth it!!

Aeroman.

 

The leaks are from the reservoir seals and the dp switch seal. The flare fittings were fine. My plan is to remove the unit and return it for another one. If this one leaks (it would be the third rebuilt to do so) I am tempted to use the Raybestos MC 390216 retrofit master cylinder instead. It appears to use the later model master cylinder that I have installed on my 1993 Aerostar.

 

IMO. I tried to repare old MCs... Clutch and brake. but seals go out working 1/3 of usual life time of new MC. sometimes they fail during first year. the only way is to rebore and resurface... but this is long story. So I think no reason to rebuild old cylinder - just replace, it is safety thing.

 

sounds like your reservoir is deformed from age/heat or has a hairline crack at mount area

the 91 model year Aero used 4 different MCs.
before 2-91 and after 2-91 used different MCs
the shorty aka standard and the EXT lengthened vans used different MCs, EXT has larger bore.

do you have the original part number from your original MC?
or date build of your van

Rayb shows that 390216 as being an OEM replacement for standard length vans aka shortys from 2/91 for the 91 model year
[Brake Master Cylinder]
Professional Grade;From 2/91;Standard Length Van MC390216

does your original MC look like this?
MC390216 comes with proportioning valve and lines

 

++
Cardone rebuilt MCs work 1/2 of genue new MC life.... Cardone use better seals, then stock cylinders have, but bore surface is worn and....

I tried to repare slave cylinders, clutch slave, air dries, master brake valves, pressure limoters, ABS modulators, all Wabco ore Bendix make parts and I know well, they fail 1-3 years later, but stock parts fail when truck is 5-8 years old.... Owners think that they save money, but.... IMO, there is a reason to rebuild ECMs and other electronic units becouse their reliability does not depend on how many miles they work. What kill electronic units? answer - bad wiring, overvoltage, overheating, corrosion, may be vibration, but they are well vibro-protected. the only thing that may fail becouse of too long time of use - electrolithic capasitors, that's why i replace them every time I open unit....

What MC reliability depends on? how many times and with what force U press brake pedal, how good and how old fluid? how hot is under hood and how hot is fluid when you stop on red? when seal fail and MC is leaky there is internal metall to metall contact causing scratches that wear new seal. so... to rebuild MC.... is to proling its life for less then half of its normal lifetime.

 

pablo is right, reboring/polishing alum. MCs is a lesson in futility. not so with your old school cast iron MCs unless they were pitted too deep from water contamination corrosion.

the home DIYer does NOT have the tools and the cheapy overseas rebuilders don't invest in the high diamond hone tech equip required to bore/polish alum MCs to perfectly round and less than 10 RA finish

get a new pressure differential switch, the old one is probably split or worn threads. will never seal.

 

The Aerostar I am attempting to repair is a 1991 4.0 ext built in April 1991. The Cardone unit specified for this vehicle comes with metric threads that do not match up with the standard fittings installed on the van. In order to resolve that problem, after consulting with Cardone's tech support, I went forward with the Cardone unit specified for the late 1991 non-ext Aerostar. Should I have second thoughts about this plan? Would this possibly be the cause of leaks from the reservoir and dp switch connections?

Both the old master cylinder and the rebuilt Cardone units for all 4 1991 applications look identical. And they look nothing like the Raybestos unit pictured above. But I knew that. The Raybestos unit is a retrofit using a master cylinder resembling the one installed in my 1993 Aerostar. Has anyone installed this unit as an alternative to the rebuilt originals?

 

Hey Guys:

Hope everyone is well?

Have not been on here in a very long time.

I did a search and I saw this thread, close enough..

Brief history.

1989 Aerostar ext anti-lock rear brakes

The power booster went bad a while ago, a long while ago..

Then it stopped working at all..

So i replaced it.. That was fun, all that room to work!!!

I am using the old master cylinder, which seemed to be working fine.

I would love to bench bleed this thing, but I can't find any local places that sell the bench bleed kit..

Here is my question(s):

Do i need to Bench Bleed the MC?

If i don't what is the proper way of pedal bleeding a MC

If i do, how can i make a Bench bleed kit?

thanks so much in advance.

Paul

 

I'm not sure if the Aerostar uses it, but Ford's fast-fill MC allows reduced initial pedal travel (low leverage) and then increases leverage when the pads engage. It was used in a lot of cars in the mid to late 80's. It uses a stepped MC piston, which likes to trap any air that gets into it. Bench bleeding allows you to turn the MC in different orientations to let the air out. If you were to do this with the MC installed, you would have to do a combination of pressing down on the pedal, hold, have a friend jack up one end of the van, release, then jack up the other end of the van to achieve the same thing. (I forgot the exact sequence.)

To bench bleed without the "kit", just get a couple short sections of brake lines with the proper fittings to screw into the MC ports. Bend the lines so they dump back into the reservoir. Then hold the MC in your bench vice, and use a suitable device to push the MC piston like the pedal would normally do. Again, at some point, you would have to tilt the MC forward and backward while pushing on the piston to get those last little bubbles out.

Then, whether you bled on the bench or in-car, you have to carefully replace those short loops of lines with the car's lines (after installing, of course). In the process, it's entirely possible to introduce air into the MC again.

 

xlt4wd90:

Thanks for the advice. Funny after I posted my question, i was wondering if i could just use brake lines...

Thanks again

 

I think i may just buy a new MC... had a Heck of a time finding the proper Power booster for it, but did find it on the Advanced Auto site.... It was for a 1991 and mine is a 1988... The MC looks like the one above.

Assuming that would fit my 1988 Aerostar ext??

Thanks