Low line pressure in a transmission is caused by a number of different factors. The most common factor is the age and wear on the components within the unit, such as seals, clutches, bands and valves which can all begin to deteriorate over time leading to decreased performance. Other problems that could be causing low line pressure are worn or damaged solenoids, vacuum leaks, blockages in the hydraulic control circuits or worn-out pump internals. Poor engine operation can also lead to lower than normal line pressures due to inadequate fluid flow from lack of power or improper valve timing resulting in insufficient volume being delivered through the system. Even leaking coolers and lines can produce excessive heat inside your transmission which causes fluids to boil off leaving it with less lubrication and thus result in reduced line pressures as well. Finally, if your vehicle has recently been serviced or rebuilt it’s possible that incorrect parts have been used during assembly resulting in poor performance when you try using them on an older model car.