Cat 426 weak boom raise only

Hello, Yesterday while digging a ditch out here on the farm the boom lift on my Cat 426 backhoe became very weak, and had to be pushed up with the stick to get to the full back position. However pushing down with the boom I was able to lift the whole back end of the hoe, even at an idle. All other hydraulic functions worked well. I shut it down and tried it again this morning and the same thing occurred. Everything worked well but the lift function of the boom. I was wondering if anyone had any thoughts on what this might be. It is a late 80's hoe. It has two boom cylinders. Thanks in advance for the help!!