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 Post subject: Trouble Bleeding Brakes
PostPosted: March 23, 2015, 4:10 pm 
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I just replaced the leaking old cast iron tandem style master cylinder in my Locost with a brand new one of similar construction. Bleeding the master cylinder seemed to go well (tubes looped back into the reservoirs method.) I hooked up the lines and started with bleeding the front disk calipers - no problems. Went to bleed the rear drums and NOTHING came out. No air, no fluid, nothing. The pedal would firm up at normal braking position and still nothing to the bleeder.

What's up with that? I've been working on my own brakes for 45 years and never saw this before. I could really use some advice on where to go from here.

The system is all OEM (Ford Pinto) from the calipers and wheel cylinders up through the distribution/proportioning block and up to the master cylinder. I did change to a slightly different master, a Speedway unit, but it is supposed to be a Ford style for disc front & drum rear brakes.

The distribution block has a sliding shaft that cuts off the front or back if a leak is detected, but that is supposed to reset itself during bleeding. The only thing I can think is that either the MC is defective or that internal valve is stuck in the "cut off the rear" position. I've been thinking of ways to unstick it (hit it with a vibrating engraver or tap it with a hammer?) but I don't know if that is even the problem - it's just all I can think of that could be causing this. I could bite the bullet and pull it out to clean and check it, but what a nuisance.

Anyone have experience with this kind of problem? Any solutions or suggestions?

Thanks in advance,

John


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PostPosted: March 23, 2015, 4:51 pm 
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John,
You might try this link http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=8&cad=rja&uact=8&ved=0CFEQFjAH&url=http%3A%2F%2Fmembers.boardhost.com%2FMustangSteve%2Fthread%2F1352514449.html&ei=CXsQVZSdCsupgwSLmYPQCw&usg=AFQjCNEDSMJzU854I6NwDzsGmzjmNcjViA
Walt


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PostPosted: March 24, 2015, 3:51 pm 
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Walt,

Thank you for the pointer. It's a good general reference for the operation of a proportioning valve. I have the original 1976 Pinto shop manuals and they give a lot of good information and a trouble shooting guide. Too bad they don't include a problem like this.

John


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PostPosted: March 24, 2015, 4:03 pm 
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Last night I took the switch off of the valve and tried to peek inside. As near as I can tell using a little mirror, the diverter valve is centered. The two terminal switch turns out to be a one terminal switch with two posts. The switch operates by having the spring loaded post at the end providing a ground to the wires if the diverter shaft gets pushed up against it or compresses it. I guess it's "wire-ORed" with the parking brake handle switch.

Tonight I'll try opening one of the front caliper bleeders at the same time as a rear cylinder bleeder and see if the additional travel on the pedal makes a difference.

John


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PostPosted: March 24, 2015, 4:24 pm 
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I hate doing armchair mechanics but a couple of things come to mind. Firstly its a possibility that the pushrod into the master may be holding the piston slightly depressed and not allowing it back to draw fluid. The piston in the master would only have to be moved a few mm `s to cause this. Worth checking as you have changed it for another type. The second thing is try to eliminate the master by bleeding off fluid from slackened pipe unions at the brake master. If there is no fluid at these you need look no further.

Bob

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PostPosted: March 25, 2015, 7:10 am 
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i had a similar problem on an old truck once. A rear brake hose blew out. After replacing it, we couldn't bleed it, even after reseting the prop valve. We had to un-bleed the front brakes, bleed the rear then bleed the front. I think what happens, is since there is no pressure in the rear, the prop valve cuts it out as soon as you pressurize the system (i.e. safety mode)


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PostPosted: March 25, 2015, 3:42 pm 
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bob wrote:
I hate doing armchair mechanics but a couple of things come to mind. Firstly its a possibility that the pushrod into the master may be holding the piston slightly depressed and not allowing it back to draw fluid. The piston in the master would only have to be moved a few mm `s to cause this. Worth checking as you have changed it for another type. The second thing is try to eliminate the master by bleeding off fluid from slackened pipe unions at the brake master. If there is no fluid at these you need look no further.

Bob


Bob - Good idea, I'll take a look at it tonight to be sure. I think the pedal is moving all the way back since the brake lights are turning off.

Thank you for the suggestion,

John


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PostPosted: March 25, 2015, 3:48 pm 
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Sean in CT wrote:
i had a similar problem on an old truck once. A rear brake hose blew out. After replacing it, we couldn't bleed it, even after reseting the prop valve. We had to un-bleed the front brakes, bleed the rear then bleed the front. I think what happens, is since there is no pressure in the rear, the prop valve cuts it out as soon as you pressurize the system (i.e. safety mode)


Sean - I was thinking the same thing so last night I opened the front right caliper bleeder at the same time as the rear left bleeder. Still no luck. The pedal would go clear to the floor, lots of fluid left the caliper and still nothing at all out the rear wheel cylinder. I think it's time to deal with the proportioning valve.

Thank you for the suggestion though,

John


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PostPosted: March 25, 2015, 3:59 pm 
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:BH:
So last night I tried opening a front bleeder and rear bleeder at the same time. The front pumps like a fire hose, the rear like a dry well. Very disappointing. So all I can think of now is that the problem is the proportioning valve.

Time to get careful with the vocabulary here. There are a zillion valves associated with brake systems and I want to be careful with the description of the next step. The next step is to remove the distribution block with its built in differential and proportioning valves and to disassemble and clean the proportioning valve. If the proportioning valve is damaged or dissolving, perhaps it can be removed from the block and an external one added in line with the rear brake line.

This will be a little messy, but completely replacing the block with a new aftermarket combo unit would almost certainly mean rebending (and possibly replacing) the tubes connected to it.

Wish me luck,

John


Last edited by Locost_Johnh on March 26, 2015, 3:41 pm, edited 1 time in total.

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PostPosted: March 25, 2015, 11:48 pm 
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Hi John

I would leave the front calipers charged with fluid. This is how I would trouble shoot your problem. If you pedal is hard, get someone to put slight pressure on it and crack the rear brake line nut at the master cylinder - (a)pedal stays hard, problem in master cyl; (b) pedal goes down and you get fluid past the line nut, tighten nut. Release and press the pedal again - loosen rear brake line nut (from MC) at proportioning valve, pedal should go down and you will have fluid past the nut, tighten nut. Press pedal again - loosen rear brake line nut going out from proportioning valve to rear brakes(a) pedal hard and no fluid, problem in valve; pedal goes down and fluid past nut, tighten nut, keep going to the rear brake hose and do the same again, keep going to the rear brake cylinders.
If there is no fluid coming out of the proportioning valve, put slight pressure on the pedal and crack the front brake line nut at the proportioning valve, this should cause the rear line pressure to shuttle the valve back to neutral.
As it sits now I would also ohm the proportioning valve switch to see if its open to ground (normal) or closed to ground (valve shuttle to one side).

Just some thoughts.....

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PostPosted: March 26, 2015, 3:31 pm 
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horchoha wrote:
Hi John

I would leave the front calipers charged with fluid. This is how I would trouble shoot your problem. If you pedal is hard, get someone to put slight pressure on it and crack the rear brake line nut at the master cylinder - (a)pedal stays hard, problem in master cyl; (b) pedal goes down and you get fluid past the line nut, tighten nut. Release and press the pedal again - loosen rear brake line nut (from MC) at proportioning valve, pedal should go down and you will have fluid past the nut, tighten nut. Press pedal again - loosen rear brake line nut going out from proportioning valve to rear brakes(a) pedal hard and no fluid, problem in valve; pedal goes down and fluid past nut, tighten nut, keep going to the rear brake hose and do the same again, keep going to the rear brake cylinders.
If there is no fluid coming out of the proportioning valve, put slight pressure on the pedal and crack the front brake line nut at the proportioning valve, this should cause the rear line pressure to shuttle the valve back to neutral.
As it sits now I would also ohm the proportioning valve switch to see if its open to ground (normal) or closed to ground (valve shuttle to one side).

Just some thoughts.....


Perry,

Sounds like a good process. I did most of that and finally concluded that the proportioning valve must be plugged. (See the next post)

John


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PostPosted: March 26, 2015, 3:40 pm 
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I haven't gotten things put back together yet, but I did get the distribution block removed and disassembled. The parts seemed in pretty good shape (o-rings and rubber seal) for being nearly 40 years old. But FILTHY DIRTY! :ack:

After brushing everything with clean fluid, blowing out the crud and reassembling it there is clear passage for fluid between the rear inlet and outlet. It won't be until the weekend before I get all the lines hooked back up and test it, but I'm confident the problem has been resolved.

John


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PostPosted: April 1, 2015, 7:55 pm 
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Well, that proportioning valve really needed cleaning and I'm glad that was taken care of. Unfortunately it wasn't the problem. :BH: :BH:

Disconnecting brakelines from the proportioning valve and working toward the rear wheels, the fluid flowed right up to the rubber hose going to the axle. Nothing made it past that hose.

With the hose out of the car it's clear the steel fitting is rusted inside and completely plugged. And I'm confident that is the problem. A new hose is on order.

John


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PostPosted: April 2, 2015, 2:30 am 
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I was going to say the rear calipers just looked like calipers but were full of rust. It didn't seem helpful though and I didn't want you to think I was giving you a hard time. So it turns out the hoses only looked like hoses. Maybe that's why some of the those 30 year old rocket engines they've been using sometimes don't work so well either. :rofl: Good news, you'll be on your way soon. :)

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PostPosted: April 6, 2015, 4:05 pm 
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I'm on the road again! New hose fixed the problem. :cheers:

Lesson learned: steel brake fittings can rust enough in 10 years to block all fluid flow.

John


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