Bleeding dual system brakes & how do they work?

[Lynn Kirkpatrick]

I have tried bleeding the brakes on my '68 MGBGT. BUT, if I use the usual method where someone pumps and holds the pedal while I open the bleeder, the pedal doesn't go down. It's the same for the fronts or rears. They bleed OK if I use pressure and backfill from the bleeder screws, but it's messy, as it seeps out around the loose bleeder screw. When I first bought it 10+ years ago the warning switch gizzy was frozen. I took it apart got it working again. Is it likely that seized?

I had myself thinking that with 2 separate systems, the old pedal method was impossible. Since only one system is open when bleeding, the other would still hold the pedal. Is there enough movement in the warning switch gizzy so that the pedal will move and each side can be bled individually? I have read in other manuals with similar systems to pump the pedal.

The real questions are how do these systems work and why won't the pedal go down with the old pedal method?

Thanks!!

 

[TomMull]

Lynn, Try this. https://www.youtube.com/watch?v=HPXBDuRgxG0
Tom

 

[DrEntropy]

Bench bleeding the M/C is a good idea but in this instance doesn't seem to be the real problem. Sounds to me like the shuttlecock in the switch has shifted.

 

[PAUL161]

I agree with Doc. Very possible the shuttle is off center. Here's a view of how it works and the good news is, their reparable. With the shuttle off center, the light should come on,(If Working) and one side of the system, front or rear will be hydraulically locked off, which is the purpose of this safety device. PJ

 

[DrEntropy]

And another issue would be the flex hoses (3 on an MGB). they can swell internally and block fluid flow. But it'd be rare that all three would be gone at the same time. I'd physically check 'em anyway. You may have two issues simultaneously.

 

[Lynn Kirkpatrick]

Thanks for the ideas, guys. The red warning lite is not on, but I need to check to see if it's working right (connect it to ground?). I will also try to look into the shuttle valve to see if it's centered.

I replaced the front hoses a few years ago. Then last year I replaced all of the hard lines ahead of the rear flex line. The hard lines on the rear axle looked good with no pitting, etc. The hose "looked" OK, but didn't get changed. I have a new one, so I'll swap it. It hasn't been towed since I've had it (knock on wood), so no wrecker hooks have pinched the rear hard lines.

Last year I changed all of the springs in the rear drums, just in case they had given up. The old ones were pitted and rusted. That didn't help.

I hope to get into this in the next few weeks.
Thanks