I have a L-4400 and the FEL lifting capacity seem low when I purchase my tractor used. I'm sure that your L-3400 is setup the same way. A quick coupler and reducer to screw the gauge cost about $ 40-50, I already had the guage, so it was worth every penny for the results.
So, I did a little research and found on Tractorbynet.com, a forum on how to increase the Hydraulic pressure setting by replacing the manufacture washers with a larger one and checking your pressure increase by placing a psi guage on your FEL quick coupler located above the control handle. I had to disconnect the Hydro. hose going to the FEL, crank the tractor and simulate the bucket roll to view the operating psi 2495. It was simple and I increased the psi from 2495 psi to 2630 psi. This setting was base on the manufactor's max. setting psi for this model. Make sure you find out what the psi setting is for the L-3400. I bought a quick coupler fitting at the dealer and asked what the max setting was for my Hyd. relief system, went to the the local hardware store and bought a 5 cent flat washer about the size of a dime, alittle smaller and about a 16th in thickness.The shime washer that are in the pressure relief block are 2 stainless thin washers.The psi block is located below the floor panel next to the hydro. filter. There is a plug in the bottom that will have to be removed, it has a spring and maybe a ball bearing, along with the washers. Remember that I set my psi to whatever the max. operating psi was set by the manufacter and do not exceed that setting. The relief valve is design to relieve at the factory setting, but it's best not to pop off at high psi. I have been operating my orange BeBe for some time now with this setting and feel very please with the way it operates now, alot more lifting capacity. On that web site they had picture and detail instructions on how to increase the psi. I didn't use the shop vac and had little Hydr. oil leak out. Just have your washer ready and remove the two and put the one you bought in. Check your psi on the guage, hopefully it will be close enough to the max setting. It has worked for me and others that are satisfied, hope that this has helped answer your problem.