Hey all,
I recently replaced the MC and powerbooster in my 1964 F100 and have a question about the proportioning valve. The truck was converted to front disc brakes by a previous owner and they installed the dual reservoir MC and the booster. The old master cylinder had the larger port in the reservoir going to the rear drums. The new MC has this larger port going to the front disc brakes. Now, I've found that the powerbooster push rod is very finicky with adjustments. If I have any preload at all on the master cylinder, the front brakes drag and then lock up when they get hot. I've become pretty adept at jumping out on the side of the road, quickly removing the master cylinder, and adjusting the push rod in when this happens. The problem is, my brakes are a bit soft when I back the push rod off. I've completely bench bled the master cylinder and bled about a full bottle's worth of brake fluid through all four brakes on the truck. I know there is no air in the lines, yet the brakes are still very touchy with the smallest bit of preload on the master cylinder. I'm wondering if the proportioning valve needs to be adjusted to send more pressure to the rear brakes or if I need to maybe switch the front and rear brake lines at the valve. Any thoughts? I appreciate any and all help anyone is able to give me with this. Thanks.

 

I have had this same issue on my 60 Mercury wagon. I converted to discs about 20 years ago. I have replaced the MC twice and each time it is a PITA to adjust and get it just right so it doesn't lock up. On my 64 F250 I used the Wilwood 1" bore compact MC and prop valve kit. No booster. Ive never had the issue with it. I honestly think it has to do with the quality of the Wilwood vs the more generic type. Did I just get lucky and adjust it right first time? Maybe. The next time the wagon needs a MC, I will be going with the Wilwood. just my $.02.

 

There shouldn't be any "preload" on the master pushrod. There should be a tiny amount of clearance. They aren't difficult to adjust.
P2 combo valve is set up to distribute brake fluid accurately to the front (disc) and rear (drum) without needing adjustment. Which master cylinder and combo valve you use and how it is plumbed is critical.

 

Something is off. Can you post some pictures of your setup? Use a push rod adjustment tool like and set the length correctly, no preload. The pedal should feel hard as soon as the pushrod applies pressure in the MC.

 

Yes! This is the only tool you will ever need in order to make that critical adjustment.
I'm a working mechanic and use this every time I install a master cylinder, cause I know that it will be correct.

 

I ordered that tool. It'll be here tomorrow. After it arrives, I'll use it to adjust my push rod and I'll take pics of the whole setup to show you what's going on. Thanks for everything, so far, guys.