I just acquired a very nice 1990 CV LX with 33,000 original miles. It is very clean and hasn't been driven much in the last decade or so, and I've got the records to prove it. I've done a complete exhaust system from headers to tailpipe and various other tune-up-related items. The car runs, drives, and shifts very nicely now.



The one problem I'm trying to sort out now is the brakes.

Ever since the test drive, the brakes have behaved a little weird. The first thing I noticed is that the rear brakes lock up under moderate braking. I'm able to control it, but it's an issue. Also, the brakes were spongy with no external fluid leaks. I bled the system thoroughly with no change in brake feel. Considering the car's age and lack of driving/maintenance in the recent past, I opted to install a new master cylinder, front calipers and pads, and brake flex hoses up front. Despite bench bleeding the master cylinder and properly/thoroughly bleeding the complete brake system after the parts install, I still have a spongy brake pedal and rear brakes that lock up too easily. I've adjusted the parking brake and rear brake shoes and they appear correct.
I've got wheel cylinders, shoes, drums, and a rear brake hardware kit including springs on their way that I plan to install for good measure, but I'm frustrated with the spongy pedal. It also seems the brake pedal wants to sink very slowly, but I still don't see an external leak. What's the likelihood the new master cylinder is bad out of the box?

Any other suggestions about what to do to fix the brake issues described above?

The last point of concern I have is the brake pressure control valve (AKA proportioning valve) that screws into the master cylinder. If it has gone bad, that might explain why the rear brakes want to lock up by allowing too much hydraulic pressure to the rears. Does anyone have a part number for that particular part? I can only find the ones for the trucks/vans, and I don't know if they're the same part for our cars. I've included a picture of the part in question below for reference.



Thanks in advance for the help.

Nice car. Congrats.

Just FYI, the only way to get a new of those little valves is to buy a new Ford MC. Of course there are alternatives (Wilwood I think), but if you want the original set up, they come with the Ford MC. I bought one (just to have a spare) 1 or 2 years ago, from ebay. They are rare, but not nonexistant.

Yes, of course! The PN is FOAZ-2140-B

And I have more: if you want to install a new MC for your 1990, here´s someone in ebay who´s selling 2 of them:

https://www.ebay.com/itm/Genuine-Ford-Master-Cylinder-FOAZ-2140-B/322986166867

In the pics you can see the (in)famous valve lol

I'm assuming that your car has the 5.0 engine, because the 5.8 use a different MC.

That´s the seller that I bought from the one from my 1990 LX.

Thank you! Yes, I've got the 5.0L, and I just ordered that master cylinder from the link you provided. I'll feel better having that as a spare, for sure. If I had found that master cylinder with the valve included, I would have gone that route rather than the new master cylinder I just put on that required I transfer the old valve over.

The rest of my brake parts have arrived, so I'm going to get them all put on, the system bled, etc., to see if my problems go away. The way I see it is that brakes maintenance is a good idea for a 30-year old car that has averaged less than 1,100 miles a year anyway.

You're welcome!

As a side note, I had similar problems with my car, but she was locking the front brakes. I replaced rotors and drums, brake pads and shoes, the hidraulic hoses, and some other little components, and no more problems with locking the brakes. And still has the original valve. However, I had to replace the original MC some years after that brake job.