Master cylinder trouble shooting
 

So I am trying to finish up a disc brake conversion on my 75 f100 4x4. I have installed all the seemingly appropriate parts from a 79 bronco. This includes the master cylinder, proportioning valve, and of course the disc brake spindles on the front axle.

So I get everything plumbed, and go to bleed the system, and only the rear brakes bleed. I checked the lines from the proportioning valve up to the master cylinder, and found there was no pressure for the front brakes coming out of the master cylinder; I determined this when I cracked the fittings and had the brakes pumped... No fluid came out.

I am thinking there are three possibilities; bad master cylinder, some sort of adjustment is out of whack, or the 75 brake booster is not compatible with the 79 master cylinder. Which of these is most likely?

Did you pull out the metering pin on the prop valve when you attempted to bleed the front brakes?

http://i249.photobucket.com/albums/g...-bleeding1.jpg

The simple answer is no. I knew there was something to do with that but wasnt aware it had to stay out. I will see if I can grab the meter valve bleeder and try it. Still, nothing came out when when I unthreaded the fitting; whether the valve was out or not shouldnt fluid come out of this?

None the less we will do this just to rule something out.

Dumb question... Can I assume they have this at the parts store? Can't find it online.

I too would expect to see some fluid, but i'd still try bleeding with the metering pin pulled out.

You can just use a pair of needle nose pliers to hold the pin out.


https://www.ford-trucks.com/forums/1...valve-pic.html

Ok so I got someone (my wife who was thrilled to be of service) to help with the bleeder valve. It made a huge difference although the rear brakes are doing more of the work. I think I have a bit more bleeding to do but wanted to double check the plumbing. Does anyone have a brake line schematic; specifically around the proportioning valve?

You didn't mention this so i figured i'd ask, did you bench bleed the master cylinder?

No. Not familiar with this.

Just watched a youtube video on this... I will try it tomorrow. After driving it a few miles, I feel better about everything as its all working, just not as well as I think it should. Perhaps its just a matter of bleeding it some more and even doing one of these master cylinder bleeds.

Bench bleeding the master cylinder is a MUST, you'll never get a solid brake pedal until you bleed all the air from the MC.

That makes sense... It was without fluid for a while. I will post an update later.

Good deal, after you bleed the master you should see a huge improvement. :-X22

I did see the improvement you mentioned. I only had the help of my 8 y/o daughter so I couldnt do the rest of the system but the pedal felt so much better. Bleeding the whole system requires 3 so its a little more of a coordinated effort. This did address the issue of no pressure coming from the front port on the cylinder.

Another question. Both this new cylinder, as was the case with the old cylinder, have turned brand new fluid dark and cloudy. Is this normal? It doesnt seem to affect the braking. No mater how much fluid I replace, this issue remains constant.

Are you using a new unopened bottle of brake fluid or one that's been opened before and sitting around awhile? brake fluid's like a sponge and pulls moisture from the air and will appear cloudy if it's contaminated.

Always new. Once it leaves the bottle or gets taken out of the system it gets disgarded.