New old '83 F250 in the North

ratdude747 wrote

Working on a brake repair/upgrade for my 1984 using a later model (Bricknose/Aeronose) master cylinder. Right now I'm getting my butt kicked when it comes to brake bleeding (Long story short: Vacuum bleeders suck, waiting on a pressure bleeder to come in). In the mean time though, I do have some information to share.

First off, Raybestos makes a new master cylinder for the later application that includes the residual pressure valve, the reservoir, and the reservoir level sensor, specifically MC39634:



As of this writing RockAuto has them for $33 before tax and shipping (no core)... which pending testing, is a great deal.

One thing I plan to (eventually) do is wire the reservoir level sensor into the brake warning circuit (parallel to the existing proportioning valve switch). The connector to do this is Motorcraft WPT-446; I de-pinned the center terminal (which on my pigtail was double wired) and used the outer two pins. The third pin is tied to one of the outer pins; this is used on later model vehicles to tell the ABS controller if some twit has unplugged the sensor to bypass a low level condition (which the ABS controller needs to know about; activating ABS with a brake line failure is a great way to make a bad situation worse!).


Before going this route, I didn't know the Raybestos MCs came with all the trimmings (RockAuto's pictures showed there not being a residual pressure valve) so I opted for a used eBay Ford unit... which was a complete loss. Nothing worked... it was seized on arrival and the front brake circuit wouldn't bench bleed. Also, the reservoir sensor was toast... more on that below.

If one needs to replace one of these reservoir sensors: you'll read online the only fix is to replace the entire MC (or the reservoir at least). Not technically true, as there are two options:

1. Find an NOS sensor. It's Motorcraft E6DZ-2L414-A, usually found on ebay for around $70 (way more expensive than a Raybestos MC with a new sensor, but if you're a purist dead set on a new sensor, that's the way you'd need to go)

2. Rebuild the sensor. The sensor is merely a potted reed switch; through some hacking away, I was able to dissect my faulty sensor (which seemed to have a bad solder joint, but I broke the switch tube):





A better way to do this may be to boil the sensor in a pot of water; the potting compound loses it's adhesion and softens with just a little heat, which would probably allow one to yank the tray out of the tube and pick off the compound without cutting or scorching anything. I made the mistake of using a MAPP torch on mine to soften the compound... even lightly used, it melted the plastic tray.

The switch tube I used to repair mine is a 14mmx2mm 0.55A unit:



If I were to use this switch, I'd pot it with some hot glue... but since I have a new one (that works great), I'll use that instead!

Master cylinder installed (I had to ditch the line bracket and re-bend the lines as the front and rear brake circuit positions are switched):



Now if I can get the darn things to bleed...