Air dryers for hand pump

Just add a filter to the line. Like this one 

https://www.aliexpress.com/item/Small-Filter-Used-for-Hand-Pump-PCP-Pump/32662162479.html?spm=2114.01010208.3.18.waYjkB&ws_ab_test=searchweb0_0,searchweb201602_5_10065_10000073_10068_10000077_10000074_10000132_10000033_10000030_119_10000126_10000026_10000129_10000023_10000123_431_10000069_10000068_10060_10062_10056_10055_10000062_10054_10000063_10059_10000120_10099_10000020_10000013_10000117_10103_10102_10000016_10000114_10096_10000111_10000056_10000059_10052_10053_10050_10107_10051_10106_10000097_10000094_10000091_10000007_10084_10000101_10083_10000100_10080_10000104_10082_10000045_10081_10110_10000108_10111_10112_10113_10114_10000089_10000086_10037_10000083_10000135_10000080_10078_10079_10077_10073_10070_10000036_10122_10123_10121_10126_10124,searchweb201603_2,afswitch_1,ppcSwitch_3,single_sort_1_default&btsid=5c9b6cb3-9f27-4ab9-9743-fd7f263ac6fc&algo_expid=25ef506d-7190-4aba-bb77-03133ba0fb43-2&algo_pvid=25ef506d-7190-4aba-bb77-03133ba0fb43

Have a look at :
http://www.airriflepump.com/acatalog/Air_Rifle_Pump_Universal_Dry-Pac_Kit.html#SID=11
and
http://www.airriflepump.com/acatalog/MK4-Dry-Pack-Kit.html#SID=11
These are for Hill pumps, but it might be possible to adapt. Look around on their site you will find diagrams.
The one mentioned by Erik is an on line filter not an air dryer (unless I am missing something)
Cheers, Muff

I have the air drier on my Hill pump and the beads, supplied by Hill, never seem to change color (they remain white). Not sure if the air in my house is that dry or if the beads just aren't removing any water. I have had to recharge (dry) the beads on my shoebox which are different purple (dry) to pink (wet) and maybe more hygroscopic. 

I used air compressor in line dryers I got from napa. They came in a two pack and with a brass adapter worked great for me when I was pumping!

You can call Sun Optics, the manufacturer of the Benjamin pump and request an air filter for the hand pump. If I recall it was $25.00, it works pretty well and you can open it up to dissipate the moisture trapped inside when it is saturated.

My Discovery was having excessive moisture issue in the airtube that caused it to rust right around the o-ring seals that required an airtube replacement and so far the moisture seems to be working well.