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Thread: Brake Shuttle Valve - Install, use, and bleeding.

  1. #1
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    Default Brake Shuttle Valve - Install, use, and bleeding.

    I just finished a total brake system install, which included a 2 circuit/line brake system and a brake Shuttle valve. There wasn't much online resources about this valve...in fact many online users deleting this valve because of complexity or ignorance. But it's actually an important safety feature and not that much of a mystery, when you get some key points. So, I thought I'd post here...since Google dumps you on RoversNorth forums 9 out of 10 times. Maybe this helps someone...

    PDWL/Shuttle Valve - What is it?
    The Shuttle Valve is a common valve in hydraulics systems where you can have 2 lines and if one line looses pressure, a "shuttle" plunger moves over and blocks the depressurized line. It makes it so that all your brake fluid doesn't squirt out onto the concrete if you tear a brake line. Car manufacturers use this feature to keep at least HALF (front or back) of your brake system working in a catastrophic leak. It makes a "2 circuit" system possible. So, deleting it just makes you go back to a 1 line system...and risks your brakes going out. Not great. It's worth having, and not that difficult.

    Another name for this valve on Rovers is PDWL Valve for Pressure Drop Warning Light. That's because it has a switch on it that can be hooked up to show you when it has been tripped. The usefulness of this switch is debatable(you know when the brakes don't work very well!) but that is a SECONDARY benefit of this valve. Some people think the only thing the valve does is operate the light switch...but that's wrong. The valve is an emergency shut off for half the hydraulic system to keep pressure. The light switch is a bonus.

    In modern cars, this is called a "Combination Valve" because it usually has a combination of Shuttle valve, failure switch AND a pressure regulator for different hydraulic pressure to the back wheels (for front disc brakes, back drum brakes setups where volume of fluid is different). Series Rovers have separate pressures from the lines out of the Brake Master Cylinder (more on this later) so they don't have a need for pressure adjustment in a combo valve. So, just Shuttle valve.

    See a diagram of the parts below.

    How it operates
    The Shuttle valve has 2 lines coming in the top, and 2 lines coming out the bottom. Each set of lines is an IN/OUT setup, with the valve able to block the circuit. Along the cross axis, the valve has an access port where you can service the Shuttle plug and Seals. Sometimes, there is also another hole on the other side of the cross axis. This is just a duplicate of the front lines circuit. You can plug this up with a bolt (NRC4137), or run individual lines to the front left & right brakes...Basically like a T. There is also a center hole which is fitted with the PDWL switch. This switch is also removable to give access to the Shuttle valve when pressurized (more on this later) or to replace the switch.
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    1973 Series 3, 109

  2. #2
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    Installing/Plumbing
    Installation of the Shuttle valve depends on your Master Cylinder and current brake lines. If you're replacing everything, then you have more options. But, if you're reusing your MC or existing brake lines, then there are only 2 things to consider:

    1) Which Master Cylinder?
    Like I said, the volume of fluid is different for front and rear brakes on a Rover. This is because the rear drum brakes have 1 wheel cylinder to actuate and the front either have (on 11" drums) 2 wheel cylinders or a Disc caliper(if you're going Disc conversion). In their infinite wisdom, Land Eover flipped these OUT lines for Master Cylinders in 1980.

    So, if you have a <1980 Dual line MC...the greater volume is on the out line towards the front of the car.
    https://www.roversnorth.com/parts/90...dual_power_109

    If you have a >1980 Dual line MC, then the larger volume is on the line closest to the firewall.
    https://www.roversnorth.com/parts/pl...ate_series_iii

    You'll see many different routing diagrams in the service books. It just comes down to routing the larger volume line to the front circuit and the other line to the back. When you know that, you can see them flip the lines every which-a-way to achieve that. Sometimes they cross before going into the Shuttle valve, sometimes they cross after the valve. All this crossing is to ensure the larger volume goes to the front brakes circuit...and the dang flipping at the MC causes this. Just identify your MC from the links above and follow the lines visually.

    2.) What's going on with your existing lines?
    If you have existing lines that don't reach right...or they were flipped coming out an old Shuttle valve...then you might be forced to plumb a front to back flip with the lines from the MC to the Shuttle valve. I had to do that (see pic). Again, just identify your MC type, and then visually follow the lines...ensuring the lines eventually going to the front brakes get the larger volume circuit.

    You can just see the flipped lines on top in my picture. The out lines below the Shuttle Valve(not visible) were straight forward, front going to front and back going to back. I had the >1980 Master Cylinder.
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    1973 Series 3, 109

  3. #3
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    Bleeding the system
    Bleeding a Shuttle Valve and 2 line system gets a bad rap. It's a little more tedious than a single line system, but only when it's dry or you've tripped the safety feature. And when you think about it, it's doing it's job!!! It's supposed to block that circuit when dry!!

    The first time you try to prime a dry system, the Shuttle valve will probably move to shut off one side. This can be annoying, because when you try to bleed that "shut off" line, no amount of pumping and releasing the bleeder screw will produce any brake fluid. Not even a vacuum bleeder will draw fluid. Remember, it's doing its job to keep you safe!!

    The Solution
    The Rover manual talks about a tedious process of gently bleeding the opposite line a tiny bit, while watching the PDWL on the dash until it JUST turns off. This is to know when the Shuttle plug is centered, and don't go too far in the other direction!! This is horribly non-intuitive, indirect, and a pain in the arse when you're crawling around on concrete under your car. Also, if you haven't wired the dash light...a bit awkward to do.

    I agonized over this procedure. I was about to tow my LR to a dealer to try to have them do this. No need!!

    There's 2 solutions:

    1) American car mechanics use a little tool that replaces the PDWL switch with a one-piece plastic or metal bolt that just holds the valve centered. You can get it for about $8 and it just screws in, holding the valve from tripping. Then you bleed the system as usual...and replace the PDWL switch when done. The only problem is, I couldn't find any online sources for a British version. Unfortunately, the thread size is different for GM Combination Valves to British Rover PDWL valves. But, if any forum members can link to a British version of this tool, that's the easiest way.

    2) The next easiest way is to just hold the valve open. Yup, that easy.

    It's like a 2-man bleeder setuo: have one person in the car, ready to hit the brakes. Then, go to the offending circuit. Attach a piece of vacuum hose to the bleeder screw, and put the other end in a 1/3 full jar of clean, new brake fluid. Open the bleeder screw. It might suck air, but the whole line is probably empty, so you're not hurting it. Crawl over to the Shuttle valve and (gently) unscrew the plastic PDWL switch, being careful not to drop the little ball bearing (if one is fitted). Then, put a wrench on the brass nut that the PDWL valve sits in, and unscrew the center access plug. You'll see the Shuttle valve with your own eyes. Wear some safety glasses for the next part...you don't want brake fluid in your eyes if you spring a leak.

    Use a flathead screw driver to gently center the Shuttle plug. It floats in there, back and forth, and shouldn't take much to push it back to center. Center is where the saddle part sits in the center of the access hole.(see pic).

    Hold the valve centered while someone in the car pushes down the brake pedal. Pump gently until fluid fills the dry line and squirts into the jar. Look for less bubbles. The first few times it will be ALL air bubbles, as the line fills up. Once you get nearly no bubbles, it's time to put back the PDWL switch and bleed the brakes as usual. You don't have to press hard to hold open the valve. It will want to trip when the driver presses the brake pedal. Just hold it open a little...the little bit will "leak" into the line and eventually fill it. 2-3 cycles of Pumping to the floor should do it. Keep the MC reservior filled so as not to suck air in the top.

    Once you have fluid in all the lines and the Shuttle valve centered, it's less likely to trip...and you can bleed the system as usual. Follow the manuals guide for how to bleed the brakes as usual. As always, only do this if you're confident in your skill and super safe. Triple check everything and I'm only offering "entertainment" so all liability is on you. ;P

    Conclusion
    So, now that you know what a Shuttle valve is, what it does, and how to bleed it...is there any reason not to have one? This is such a neat safety feature you'll sleep better knowing your wife or kids driving your Rover won't be risking their lives. Why not use it?! And now you can tell those folks at the next meet up how to maintain their 2 line systems instead of removing them!

    Happy Rovering!!!
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    Last edited by vlad_d; 11-30-2021 at 09:06 PM.
    ...┌───────┬──,,
    ...|______OD__|__\\_____
    ...d ..__ ......... |... | ..__....p
    └/ | \────┴──┴/ | \─┘≡
    ..../..@........................@

    1973 Series 3, 109

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