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Only my tub fauce water pressure starts great, then after 5 seconds it drops off.

It happens on both hot and cold settings.

What could be causing this?

Thanks in advance for any advice.

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    If you have an older home with galvanized pipe this is normally a problem caused by the pipes corroding and the lines getting smaller so you don't have enough area to keep the pressure up. – Ed Beal Mar 20 '16 at 22:03
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    @EdBeal Sounds right, except that 5 seconds is a long time this to continue; the excess water needs to be supplied by the springiness of the pipes. Perhaps this could happen if the outside supply pipe was restricted, and there were accumulation tanks somewhere in the house. – Daniel Griscom Mar 21 '16 at 0:28
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I think you've got debris in the lines & the tub is the perfect place to flush it all out. You'll want to turn on the closest faucet, with the aerator removed, to the Main Water Shut-off for the whole house & then turn off The Main.

You'll want to leave that closest faucet open for now & turn The Main on & off several times to make sure it itself is scraped clean & only clean water is coming out of that faucet. It's common for The Main to accumulate mineral build-up. Then, turn off that faucet, reinstall its aerator & turn The Main off.

Now, you'll need to go to the tub & remove those handles & their valves, if you have 2 of them. If you have a single handle, then that handle, its wall-plate & its cartridge need to come out. Then, shut the tub drain & door or curtain & go turn on The Main to flush everything into the tub.

That should clear everything & get you back to normal. If you're unfamiliar or uncomfortable then take pictures as you go or call a plumber to do this operation.

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After you try Iggy's answer to clear the pipes, if you still have a problem, I would suspect the pressure balancing valve is malfunctioning. That is why only the tub is having this problem.

You would then have to disassemble the shower/tub valve and at least clean it. You may need to replace parts of it.

Good luck!

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