4″ Mainline Automatic Backwater Valve

The newly reintroduced ML-FR4 ABS Automatic Backwater Valve is a great choice for retrofit situations where you have a limited amount of slope to work with.

This is a normally-open design, automatic backwater valve that permits the free circulation of air throughout the plumbing system, allowing gases to move just as if the valve wasn’t even there.

The valve will close automatically when sewage backup occurs, reducing the risk of sewage backflow throughout the dwelling.

The ML-FR4 is designed to be installed in building main lines to prevent backup through floor drains, shower drains, toilets and sink drains.

It closes automatically when the municipal sewer line or septic system starts to back-up or a reverse flow occurs.

The clear plastic lid allows for easy visual inspection and accessibility for cleaning.

The design doesn’t restrict the withdrawal of a sewer snake since the gate remains in the open position, except when a backflow is in progress.  The ML-FR4 drops 1/2″ from inlet to outlet over its 12″ installed length and is the outside diameter (male) of 4″ Schedule 40 ABS on both ends making it easy to fit either end into a female coupling or neoprene rubber with stainless steel clamp (Fernco brand type) adapter.

Patented USA 4503881 and Canada 1196835

Normal flow through a Mainline ML-FR4 ABS Backwater Valve Back Flow situation (reversal) in the ML-FR4 Backwater Valve


Mainline ML-FR4-A ABS Backwater Valve installation tips in basement floor.


Normal flow through a Mainline brand automatic Fio ML-FR4 backwater valve        Back-flow through a Mainline brand automatic Fio ML-FR4 back-flow valve

Fio Valve ML-FR4 Top View 1Fio Valve ML-FR4 Top View 2

ML-FR4 Fio Valve end view closedML-FR4 Fio Valve end view open

ML-FR4 Fio Valve cutaway view openML-FR4 Fio Valve end view open 2

ML-FR4 Fio Valve als offered with PE-2013 Access Box

Also Offering ML-FR4 FIO Replacement Parts

Clear Replacement Lid for ML-FR4 Replacement Rubber Lid Gasket for ML-FR4
Replacement Gate Assembly for ML-FR4 Replacement Float Ball ML-FR4 Replacement Gate Hinge Pin Retainer Tabs for ML-FR4

Product Code # Product Description Model # UPC Barcode Units/Case
66002 4″ Mainline Automatic Backwater Valve ABAMLFR4

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