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TR6 Brake Bleeding

Govtslug

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I bled my TR6 brakes yesterday and filled it with fresh DOT3 fluid. No problems bleeding, I it did it the old fashioned way with my helper pressing/releasing the pedal while I opened and closed the bleeder screws. I took it out for a test drive and now the pedal travels much further than before. It’s not spongy at all and I can still lock up the wheels, but not until the pedal is almost on the floor. i don’t see any leaks, what do you think made the brake pedal change? Do I just need to bleed it more? The car had a frame off restoration 5 or 6 years ago, so all the lines including rubber ones are relatively new.

Thanks!

govtslug
 

DrEntropy

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Is there a PDW in the system? If so it may be it has "tipped" to one side.
 
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Govtslug

Govtslug

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It’s installed but the wire isn’t connected. How can tell if there is a problem with it?
 

Tybalt

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It’s installed but the wire isn’t connected. How can tell if there is a problem with it?
Can you lock both front and rear or only the rear?

Remove the PDWA switch, if ramps on the shuttle piston are in the middle instead of that small diameter section between the ramps, then one side of the hydraulic system is partially blocked off.
 
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Govtslug

Govtslug

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I know the fronts lock, not so sure about the rear. When you say remove the PDWA switch, do you mean disconnect all four brake connections or the big ‘plastic’ nut on the end? I’m thinking that should be my last resort since that will put a ton of air into the lines when I reconnect it And will take a lot of bleeds. All I did was bleed the system, why would my pedal be so different? Do you think re-bleeding at the rears will make any difference? The pedal is firm, not spoungy it just travels much farther.
 

poolboy

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How about a rear brake adjustment ?
Get them right and the brake pedal won't go down very far.
But if pumping the brakes is the only way to obtain that, then you still have air in the hydraulic system.
 

bobhustead

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Check what Ken said. Then try a rebleed. When I was a BMC line mech, we would figure out whether the front or rear was shut off by the PDA . Then we would crack open the fittings on the blocked side and have someone slam the pedal and hold it, then close the fittings. The slam would usually move the piston in the valve back to center.
DO C- Do I correctly recall that there was a light that came on if the valve was shifted?
Bob
 

poolboy

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Yes on the dash the BRAKE indicator bulb would receive the FULL 12 volts and would be noticeably brighter than when it gets only 6 volts during the 'bulb test' position....The low oil pressure warning lamp being in series gets the other 6 volts in bulb test which is IGNITION KEY ON....ENGINE NOT RUNNING.
 
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Govtslug

Govtslug

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Ok thanks. I’ll try opening one of the rear bleed valves and slam on the pedal a couple times to see if it frees up the valve. I would think the rear brakes wouldn‘t need to be adjusted since I didn’t touch them, just bled them. I’ll let you know if I can get it to work!

govtslug
 

poolboy

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The PDWA might and in fact probably will decentralize if you employ that method.
UNLESS the PDWA is leaking around the nylon electrical switch that's screwed into the top of the PDWA it doesn't figure into this scenario at all..even if the shuttle is off center....
The front and rear hydraulics are separated within the brass PDWA.....UNLESS there is a leak as I just described.
 

Tybalt

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First off, picture credits to Buckeye Triumph's Tech Pages.

Please note that if you do not have the PDWA switch connected, all the talk about using the warning light to determine shuttle piston centering is moot. No connection, no light.

With these cars, when the PDWA is in place, bleeding the brakes will shift the piston and it must be recentered.

The center volume of the PDWA should be dry unless one of the PDWA seals is leaking. This means that you can remove the switch without introducing air into the system. If you pull the swicth and see brake fluid, it is leaking and you need to replace the PDWA piston seals.

There were two styles of shuttle pistons used on the TR6 but with yours being a 1970, it should be the later style. The middle portion of the piston is much smaller in diameter than the rest of the piston body. There are ramps on either side of the small diameter center section of the piston. That is where the pin from the switch would sit under normal operating conditions. If pressure is lost in one of the two brake circuits the shuttle piston is shifted which in turn pushes the pin on the switch up as it rides up the ramp. If connected, that will trigger the warning light under normal driving conditions.

If the PDWA shuttle piston is shifted forward, then you need to open a bleeder at the rear. If the PDWA piston is shifted rearward, then you need to open a bleeder on the front. (IIRC)

I have heard of people using an awl or ice pick to manually shift the piston but I have always shifted the piston back to it normal "neutral" position by hydraulic means by having a helper open a bleed valve and slowly pushing the pedal until the piston is centered, then have the helper close the bleed valve. When connected, the piston is centered when both the oil pressure light and the PDWA light glow dimly. If the PDWA light is bright and the oil pressure light doesn't glow, then the piston is not centered. The bleeding and PDWA piston centering is described in section 70.25.02 of the Bentley manual if you have one of those. If not, you should get one, it is essentially a reprint of the factory shop manual.

If the switch is not connected, then you are looking at two helpers. One to crack open the bleed valve and one that knows what they are looking for to be watching the piston as it goes to the centered position.
 

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poolboy

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When it is written "must be recentered" that is if the electrical switch is connected in order to make a recentered shuttle break the ground connection to the warning light and turn it OFF..otherwise an off centered shuttle doesn't matter UNLESS there is a leak around the threads of the plastic switch....if the O-rings or seal on the shuttle are intact there is no intermingling of the front and rear hydraulics regardless of center or not.... Its the seals, not the position of the shuttle that keep the front separate from the rear brake lines
 

Tybalt

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And here is where I have had conflicting information shared going back to ~1974 on the PDWA. I do recall one of the BL service reps saying that the later style PDWA with its rather more bulbous ends would restrict fluid flow on the portion of the brake system that had the operating pressure loss and that was why we always needed to make sure that the shuttle piston was recentered after performing any hydraulic brake work. Others have said no it doesn't. I have not had a late style PDWA body that I could section and move the piston back and forth in the bore to see if flow restriction is realistic or not.

Anybody out there in TR6 land ever sectioned a later PDWA and does it look like there could be a appreciable flow restriction if the shuttle piston was pushed well along the available bore travel?
 

poolboy

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No matter the seal type, the shuttles move the same distance...enough to raise the switch's plunger from the bottom center to the top of the adjacent 'hill'. ,,,That's as far as it goes and as long as a seal is sealing there is no intermingling.
 

Tybalt

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And that is why I asked the question since I had gotten conflicting information ~1974 and Bob's statement "we would figure out whether the front or rear was shut off by the PDA . Then we would crack open the fittings on the blocked side" in post 7 brought that back to me and had me thinking that it can indeed block off one of the circuits. But then that ran counter to what a different BL guy and a couple of other mechanics had said that it doesn't. I wasn't really concerned about fluid intermingling since it is obvious from looking at the line routing and the functional description of the PDWA that the only way to have fluid intermingle would be for both sides of the PDWA shuttle piston to be leaking and if that was the case, it's definitely time to rebuild, replace or eliminate and bypass. That center cavity is intended by devoid of fluid as neither version of the switch itself is sealed and fluid at the switch is the most common sign that the PDWA is leaking.

Then again we were at a BL dealership and our obligation was to set things back to how they were specified, so the PDWA was replaced since no seals/repair parts were in the spares system for the PDWA other than the switch which came through from the Lucas spares system and not through BL. Several years later away from the BL dealer world which was in the process of imploding, we were known to to just remove the PDWA and make up new lines for the front and rear circuits if the customer wanted that. I have replaced complete PDWA units, eliminated them and rebuilt them over the years, but never felt that I was in a position to section a later one for confirmation since what few extra ones I came across might be rebuilt and used on a TR250 or early TR6.
 

DrEntropy

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The PDW device is there to shunt a leaking/faulty front or rear side of the hydraulic system. How does opening the bleeder on the faulty portion and stabbing the pedal recenter that shuttlecock? The higher pressure of the working side will only keep the piston in the locked position.

I used a small screw-stick or other prying device to center the piston from the removed switch's opening in most cases where the seals were intact. If fluid was leaking into the switch port the PDW was resealed/rebuilt or replaced.

On my own Elan +2, I eliminated the poxy thing. Used hydraulic couplings (Oh Nooo!!! Nononoooo!) in its place. Tossed the boosters in the "spares" pile as well. A bit more pedal effort needed, but locking up the wheels is easy enuff and a much better braking "feel" and control, IMO.

EDIT: This is a good argument for an Eez-e-Bleed with low PSI to bleed brake systems. With a PDWA in the system, pedal pumping can shunt one side or the other if the pedal stabber is too vigorous. Likely the OP's cause of the problem, is my WAG.
 
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poolboy

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Despite all the talk (mostly by me) about the PDWA, I really do not believe it has a thing to do with the situation first described by 'govtslug'.....unless the PDWA is leaking around the threads of the plastic switch.
 

Tybalt

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Despite all the talk (mostly by me) about the PDWA, I really do not believe it has a thing to do with the situation first described by 'govtslug'.....unless the PDWA is leaking around the threads of the plastic switch.
While I agree there is no mention of fluid at the PDWA switch in the first post, it is something to look for as the leaks at the PDWA piston can be subtle. It probably would not hurt to pull the switch and make sure that it is dry in that well. If dry, no problem, if wet, time to think about pulling the PDWA and replacing the seals. Or use the options of replacing with a new unit or just tossing it and either use unions or make new lines to completely bypass it.

As for centering, looks like the good Dr. is another that has used the awl/ice pick technique of PDWA shuttle piston centering. Sounds like the idea of using the Gunson EEZIBLEED is a good idea as well to keep the shuttle piston centered. I've toyed with the idea of getting one of those but just haven't pulled the trigger, maaybe this is the push I need.
 

DrEntropy

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Sounds like the idea of using the Gunson EEZIBLEED is a good idea as well to keep the shuttle piston centered. I've toyed with the idea of getting one of those but just haven't pulled the trigger, maaybe this is the push I need.
The one I've had for decades as been in three shops with me, makes bleeding a system a one-man job. High praise to Gunson for a great tool.
 
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