Many hydraulic leaks are easy to find and fix. A blown hydraulic hose is pretty obvious, and replacing the hose is a certain fix. However, identifying a hydraulic fitting with a slow leak can take detective work, and replacing a leaky fitting might not be as straightforward as it appears.
To pinpoint slow leaks, use compressed air and contact cleaner to clean away every bit of sludge associated with the leaky component. Once the area is absolutely clean and totally dry, toss a handful of planter talc, baby powder or cooking flour onto all suspected leak points. Then run the machine at full rpm, cycling hydraulic functions to pressurize all systems.
Let the machine sit overnight, if necessary. Eventually even the teeniest, slowest hydraulic leak will reveal itself as discolored planter talc/baby powder/cooking flour.
Many slow hydraulic leaks are due to faulty fittings. Flared fittings mysteriously loosen, rubber O-rings rupture or metal fittings crack. During repairs it’s essential to recognize what sort of fitting you’re working with and reassemble it correctly.
Pipe thread-style fittings come in two designs: National Pipe Tapered (NPT) and National Pipe Tapered Fuel (NPTF). NPT fittings seal by wedging the sides of their threads and require either Teflon tape or “pipe dope” to seal the peaks and valleys of the threads. Galvanized or black-pipe NPT fittings are designed for fluid systems exposed to 300 psi or less and should never be used in hydraulic systems.
NPTF fittings look similar to NPT but are usually plated with zinc dichromate, which gives them a silver or gold tint. NPTF fittings seal by wedging the sides, peaks and valleys of their threads, enabling them to withstand thousands of pounds of hydraulic pressure. NPTF fittings will seal if assembled dry, but a lubricant is recommended.
Straight-thread O-ring fittings use a built-in nut and flat washer to compress an O-ring on the male fitting into a chamfer around the port of its female counterpart. To install a straight-thread O-ring fitting, back the nut and washer away from the O-ring. Lube the O-ring and install the male fitting finger-tight into its counterpart. If a 45° or 90° fitting must be aligned to connect with hoses or metal hydraulic lines, the fitting can be backed-off no more than one turn before tightening the nut and washer.
Never reuse straight-thread O-ring fittings that rattle when shaken. Rattling indicates the washer is worn.
Flat-face O-ring fittings are common in hydrostatic systems where pressures can exceed more than 10,000 psi. They use special small-diameter rubber O-rings that fit in grooves machined into the flat ends of the fittings. Traditional rubber O-rings common in farm shops are too “fat” and will not properly seal flat-face fittings.
Flat-face fitting connector nuts should wrench-tight within one revolution after they are hand-tight. A rubbery feeling, or slow build-up of resistance to tightening, suggests the O-ring is out of place.
Flared-fittings come in 37° and 45° designs and share thread pitches. Make sure you connect apples to apples and oranges to oranges when working with flared fittings. Lube the male and female portions of the fittings with hydraulic fluid, then hand assemble. On flared fittings smaller than ½", use one wrench to hold the fitting and another to hold the nut; then position the wrenches so you can squeeze them together with one hand to apply final torque.
Over-tightening flared fittings risks cracking the tapered male portion. It’s better to under-tighten a flared fitting and have to re-tighten it than to overtighten, crack it and have to replace it.
An Aeroquip rep told me if you torque a ½" flare fitting as tight as you torque a ½" nut and bolt, you better have an extra flared fitting on hand.
An experienced farm mechanic by day, Dan Anderson’s practical shop tips, tricks nd fixes are tested and true. Read his blog here.