After replacing the sand in a sw256t swimpro

Once you have the plumbing disconnected and valve removed,
Use a shop vac. Works well.
Once the sand is almost all removed, you can tip the tank over & rinse out with hose for the rest.

A pressure build up, indicates several possibilities: 1. An obstruction or closed valve after the filter. 2. A clogged filter. If your filter is equipped with a multiport valve, try the valve setting in the backwash or bypass settings and see if the pressure changes. in the bypass setting, if the pressure remains, you have an obstruction or closed valve after the filter. if the backwash setting causes the pressure to drop, you either have a seriously clogged filter or an obstruction or closed valve after the filter. Assuming the sand is less than 2 years in place, have you checked the sand condition by digging down into the sand deep and checking to see how much debris is still in it? Sand filters DO NOT backwash everything out and will build up over time, requiring a complete dump, clean and refill with fresh filtration sand. If you take the time 1 or twice a season to manually stir up the sand and then backwash repeatedly after stirring it up, you can keep the sand in place for several more years of service. If you live in an area with hard water, it is not unusual to see the sand turn hard like concrete and have to be replced after only a couple years of use. This is typical when pool water pH is high and proper maintenance of the chemistry is neglected. As to the lid popping off, this means the pressure is excessive or the clamp is damaged.

Another point to consider if the condition of the pressure gage... is the reading accurate? In most cases the reading should never be higher than 20, ideally, down around 10-15PSI on clean. Any reading higher, indicates a dirty filter or obstruction after the filter.

If there is not much pressure, the problem is with the circulation system. Probably the pump.
When a sand bed gets old the pressure goes up on the guage, but the filter doesnt do much. When there is a lack of pressure, its due to a problem with the water flow. Usually a clogged impeller, strainer basket, or skimmer basket.

The most common solution for a loss of pressure is to dissasemble the wet end of the pump, clean out the impeller, and replace all o-rings and seals, including the shaft seal. This will revitalize the performance of your pump.

Be sure it is not plumbed backwards and be sure you got a sand valve as a DE plumbs the other way.

Here is a copy of the owners guide for these pool filters. The answers to your problem should be there. Check the troubleshooting guide.

http://www.backyardcitypools.com/images/hayward/pro-series-manual.pdf

150 pounds or 6 bags

The SW256T was a marathon group exclusive. It is not standard to the Hayward catalog. You are not going to be able to go into most dealers and ask for parts for a 256 and find it. The good thing is with the exeception of the tank( which should be greenish in color) All the parts used on the filter are in the hayward catalog and are used in other filters they sell.
You will need to call hayward, and find out the part number. Id love to say it's same standpipe on the S230T or 244T but I can't be 100%

It sounds like the pump is either overheating or the thermal protection switch that senses the overheating situation is defective. If nothing else changed like running on an extension cord, etc, a small motor repair shop can get you back up and running fairly inexpensively.