Welcome. While I can't address all of your questions, I can steer you to www.tractorsmart.com. Once there, just enter the site and click on the "Info" tab. You can find tire sizing recommendations there for 4WD tractors. Just follow the guides.
While there you can also get a parts manual...
I certainly don't know of any good popular "boneyard" for sheet metal. I would suggest checking local dealers and watching E-Bay. I know that my local dealer maintains a nice inventory of used, older parts including sheet metal. Beyond some luck at your local dealer/E-Bay -- I think you're stuck...
If you haven't already done so, it may pay to inspect all the linkages between the lever and the arm on the side of your transmission. I found my linkage bent preventing the lever from moving most of the time --- the bend in the linkage actually bound up the lever. Took it apart and straightened...
My TPH files are all too big to attach so if you can send me your E-Mail address, I can forward a copy of the ASAE TPH Standards --- it's way more than you would ever want to know.
FWIW -- bolts are not shear pins and vice versa. Shear pins are designed with a task in mind -- protecting you equipment. They need to shear at the right torque. Use bolts with the understanding that you risk your equipment.
Many years ago I got a line on a cheap box of shear pins for my baler...
Or forget the weights (I know --- they cost money so you would like to use them) and just scoop up a bucket full of your favorite ballast. Dump it back where it came from once you've finished.
Try a second plug beside the first one --- you're using plugs that come with glue to be applied to the plug?? If not, you may want to try that style.
If that doesn't work, I think you'll be stuck with an internal patch. Don't think you can patch on the outside.
Not really sure about a B21 --- but, on the B20, there's a dipstick under the seat to check hydraulic fluid and next to it a fill port for hydraulic fluid. Check under the seat.
I think you're looking for the parts manual. Try www.tractorsmart.com --- click on the "Manuals" tab and follow the prompts. You'll get a download of the manual at no charge. Should give you what you need.
I have loaded rear tires. Since I don't need to worry about anything freezing, I just used water. In addition, I have a box balde that I normally leave on that hangs another 500 lbs. or so back about three feet. That combination seems to be plenty of ballast for me.
You could also buy or build...
I'll give you my "Reader's Digest" version while openly admitting that I don't know which control valve you have or how it's constructed. I'll reference my control valve and then someone else who knows yours will need to provide more help if you need it.
My control valve is a "position control"...
See Vic's counsel --- probably time to look at the control valve. Sounds like there isn't any leak at the piston so the only other logical choice is the control valve.
Question -- as the TPH leaks down does the control lever move down also??
By "front end" I meant --- hood, grill, various shields in that area, hydraulic oil cooler, hydraulic pump, radiator, and then the front cover. And in my case, all while crawling over the FEL.
Tough little oil pump to get to ---------
Obviously your choice on the size, but at that large size, 1 13/16" is only 0.002" over. I doubt that would cause any problem. But it also needs to be extra deep so maybe you're stuck with the dealer. Good Luck. Hope I never have to do that.
It's just amazing that the front end has to come off...