How do you know when the underloader valve is adjusted properly?

Hi,
If you mean the unloader valve, then its purpose is to open and allow water circulating flow to prevent heat buildup when the pressure washer is run but the gun is not engaged (triggerred) for a while.
You can hear a note change in the washer when it kicks in to allow flow. Not recommended to check as damage can occur even if it is working correctly.
Operate the washer sensibly and you need not worry. Turn it off if it will not produce a pressurized stream/spray for any period (anything over a minute.)
Peace,
Clarke

Clarke, while you are at it can you tell us what causes a unloader to not unload when you pull the trigger. It just stays on very low pressure for about 30 seconds and then suddely kicks in with full pressure.

Hi,
The function of the unloader valve is generally done by standard hydraulic principles. Since there are multiple types on pumps, not all unloaders work quite the same.
The three most common pump systems are axial, rasial and multi-stage.
Anyone familiar with hydrostatics will recognize the first two as the common types of hydraulic motors and pumps. The difference between a pump and a motor is how they displace fluid. The pump is variable and the motor is fixed. The third is a bit unusual in that one stage adds to the work of the previous stage and the most common is three stages.
Well, a pressure washer works like the motor in that it is a fixed displacement. With the trigger squeezed, the pressurized water is discharged. When the trigger is released, it is trapped and immediately begins to heat since work is still being done on it (keeping it under pressure is the work.)
In hydraulics where pressures vary, the control circuits have special valving (pilot) that assist in opening and closing the main valves. It is done by pressure changes within the control open a pilot and thereby the main. Reducing flow causes the main to shut. The pilot does not help.
Now back to the washer, it works a bit different because as I said, and I hope was understood to say, the only time the pressure goes down is when you squeeze the trigger or turn it off. Well, there are actually two types of unloading possible. When the trigger is released a pressure spike causes the first valve to open and the pump simply takes the water and re-pump it in basically a circular fashion (figuratively.) If heat builds enough and the water gets hot enough it begins to vaporize. The vapor cannot possible manitain the pressure and a valve working about the opposite of the aforementioned pilot valve cannot hold shut and opens. Commercial machines will dump the hot liquid to get the valve to shut again Cheap machines will probably be damaged internally as they all depend on the liquid to maintain the lubricity of the parts (no lubrication can occur without oil.)
Now the terminology (as if I have not already been doing that) an unloader unloads when the trigger is released and pressure spikes. Under proper conditions it loads when pressure no longer relieves it.
An unloader that does not release or delays is most often caused by internal damaged caused by previous heat buildup. It is not necessarily that if hard water is run through the washer causing mineral deposits. Most washers are not prone to corrosion, but some can be damaged by sitting for extended periods with some less than desirable water in them.
Long-winded enough? Often questions only give answers that are in the form of questions that may lead to a definitive answer.
Finally, Briggs has been on a "buying" spree for the last several years. What better way to insure the use of their engines. They bought a pressure washer company, among others, and this is the link for the glossary (could not find the tutorial they used to have; better than mine.)
Pressure Washer Terms - Glossaries - Briggs & Stratton (http://www.briggsandstratton.com/corp/glossary/pressure_washer.aspx)
Peace,
Clarke

To properly adjust a unloader ( and they are not made to be adjusted and are not a pressure adjustment, pressure is adjusted by nozzle size)

But you will need a pressure gauge. You adjust it till you reach the proper pressure for your unit, but without it causing a increase of back pressure for than 200 or 300 PSI on standard home units.

I will note many of the pressure washers you buy at discout stores, home supply stores are not made to actually reach the PSI rated on the units. In the years of working for professional washer companies, we have blown up dozens of the cheap home units trying to bring them up to their rated pressures.

I lent my pressure washer to my son. When I got it back it did not have much pressure. Turns out he turned the underloader adjustment knob thinking it was used to adjust pressure. I turned it back up some so I could use the pressure washer but need to know how to properly adjust it or should I take someplace for repair.

Hi,
Requires a gauge as mentioned in answer previous (not mine) to your question. Best guess may be best unless you have a shop you trust and do not mind the bill; most shops are not equipped or trained.
Peace,
Clarke