Siamese Pressure washers 1 unloader

Hi everyone,

Starting this season I’ve decided to run two 4.5 GPM pumps together with a Siamese kit (Same pumps, Engines and unloaders). One pump I ran last season, rebuilt this spring however its not running as much PSI as the new pump. They ran normally for the first few days with good flow and consistent pressure. I installed two new unloaders on both machines however they seem to be fighting with each other, even with the check valves installed upstream on the Siamese kit. It runs fine for a few hours then seemingly randomly acts up. The unloaders continually cycle and sound like little jack hammers going off. Both pumps run fine independently with normal pressure when unhooked from the Siamese kit.

My question is has any one installed one unloader for two pumps to run off of? I was planning to run it after the two check valves to try and solve the issue of the unloaders constantly kicking In and Out and competing with one and other.

This is a topic that is discussed every so often and always ends the same.

That set up doesn’t work. Take the two 4.5s and sell em and buy a dedicated 8gpm machine.

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What would happen if he had both pumps running to one unloader? like a zk1

Why is one of the pumps running lower pressure? Does it have a smaller motor?

Both unloaders should be set to the same pressure. This is a “weakest link” scenario. Your max pressure will be dictated by the less powerful machine. I’m guessing the jackhammering is happening from the unloaders bypassing as slightly different pressure points. Going to a single unloader should theoretically solve the issue. But I’d probably stick with a trapped pressure (vs. flow-actuated) unloader, and set it to the lower pressure. The zk1 might act unpredictably with two pumps feeding it.

But I’m in agreement with @mwpws. Invest in a 8gpm machine.

Alex good grief where have you been?


Thanks for all the Input everyone. I’m going to try the single unloaded for my next week of projects and will post an update. I’m planning to go the 8 GPM route this summer.

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Lol. I guess I just sorta lost interest? Bigger issues on my plate for awhile there. But I hope to be a little more present now.


Glad to see you back!

Yeah, @Infinity you’re an appreciated contributor on this forum.

Hey Alex and gang


Been running this setup for the past few weeks without issues, using only one unloaded for the two pumps seems to have made the difference. I’ve put approximately 50 hours on it so far with the two pumps running. I use only one pump to downstream for the time being.

No unloader hammering anymore.

Thanks for the input everyone