86 F350 460 and I just replaced the MC after pedal went to floor and brakes were locking up. $19 O'Reilly Master Cylinder was bench bled and put on WITHOUT me knowing about pushrod adjustment. Brakes kept locking and I finally read in my Chiltons where I should have checked the length.

Removed MC and re-bench bled for the fun of it and noticed a lot of fluid dripping out the forward small reservoir port before I put yellow plugs in for trip back to engine bay.

Checked pushrod length and found it was 1 5/16ths. Turned it in to try and get to the .980-.995 and the shortest I could get it was 1 1/8th. Old MC and new MC have same internal piston distance (near c-clip) to back flange and look identical. Mounted MC and re-bled brakes and got almost zero flow out of driver's rear bleed screw so I think the proportioning valve will need to be reset and I have read about how to do that. BUT, brake pedal is not returning to upright and I have mediocre firm brakes when driving but only starting a couple inches off the floor.

I have read a dozen plus threads from 11 pages of brake pushrod results so I know my pushrod needed to be adjusted the first time. I took the MC back off AGAIN to confirm that the forward reservoir fully drained of all fluid over in less than an hour sitting on the bench and is now dry. So, I am thinking my too long pushrod damaged the bore of the new MC causing this leak, but $19, so big deal. BUT,,,, if I can only get my pushrod back down to 1 1/8th and not below the inch mark am I going to have the same problem with the new MC I buy tomorrow or do I have another issue? Pushrod does not pull all the way out from booster as I have heard some little c-clip can get out of position if so.

Thanks for any insight with my too long pushrod.

 

The too long pushrod can't damage the master cylinder. What it does is hold in on the master cylinder a little bit, and that plugs off the little holes down in the bottom of each reservoir. Those little holes go directly to the front and rear brake system. As the brakes warm up, the fluid warms up and expands, so it expands back into the master cylinder through those little holes. If the holes are plugged off, then the fluid expands and starts to apply the brakes as you are driving down the road.

One way to verify if the adjustment is ok and not plugging the holes is to watch for a "geyser" of fluid to come up out of each reservoir as the pedal is pushed down. You can also push the master cylinder up onto the booster, and see if it goes all the way to the booster, or if there is some resistance that would be the rod starting to push on the master cylinder.

If all that looks good, I suspect you still have air in the brake system. Never let the master cylinder run dry when bleeding the brakes. And I found out if the rear system is dry, it takes more than one reservoir amount of fluid to get the rear system filled back up.

 

YES, you described the delayed warming condition I had to a "T". I knew after reading that on the web somewhere else that I probably had a too long pushrod and thus, forced me to actually read my Chiltons.

Great tip on watching for the little geyser flow. And, I noticed when pushing the MC up to the booster that the larger gap (which tightening the bolts back and forth took care of) turned into a smaller, but YES, I still have a gap of no more than a quarter inch if I remember right which is less than before. I understand that the pushrod should not/or just be touching. With the nub of the pushrod spun in I am still over by spec. I will get that proportioning valve reset and re-bleed the brakes, but I still think I am going to have a pedal that is 2 inches off the floor.

My wife is well instructed on topping off the reservoirs as we bleed and we never run dry as I know air gets entrained then. We check every 5 reps or so and I use and have plenty of fluid on hand.

So, if too long of a pushrod won't damage to seals or bore of a new MC, I still think I have a problem with my $19 unit if the front reservoir will drain of fluid when sitting still with no lines attached. I will swap if tomorrow since it is so cheap.

 

let me clarify a bit since I just read my post I hurried... I noticed the gap from MC to Booster changed after my shortening the pushrod. With the pushrod as short as possible at 1 1/8th" inches (out of spec by 1/8th+) I still have a 1/4" or less gap between the MC to Booster when sliding it up.

 

It is normal for the fluid to drain if you dont have the line connectd

 

I am in the middle of helping a guy with a Ford Ranger that has the same problem with the brakes dragging and locking up. After they replaced the calipers, the master cylinder, shortened the rod like you did, it still does it. I told them to crack the master cylinder lines and see if that releases the brakes when they lock, and it does. So we know the problem is at the master cylinder or behind it. Since the master cylinder is new, the last thing they are going to do is loosen the bolts on the master cylinder to booster, and see if it releases the brakes. If it does, the only thing I know to do is replace the booster.

What I told them to do on the ranger is park it on a little hill, and put a block of wood a couple of inches behind the wheel. When the brakes release, the truck rolls against the block.