Brakes Locking up ! 1969 W200, Help.

Suspension, Brakes, Tires, Wheels etc..

Brakes Locking up ! 1969 W200, Help.

Postby cometphoton » Fri Jan 13, 2017 8:51 am

First of all thanks for taking the time out to help me.
If it wasn't for this form I don't know what I would do. So thanks again.

My 1969 W200 brakes are locking up. "It seems to happens after they warm up". We changed all wheel cylinders and the Master cylinder and did Multiple bleeding. I was thinking it was the park brake as it seemed to happen when I used the parking brake; but not all the time. Now I think this is not the issue as I am not using the park brake and after we drove for a while using the brakes the problem croped up. Its a dusey.

So I have decided to leave the parking brake off for a while making sure I did not use it.
Well when I went for a ride; about 3 miles yesterday and I was using the brakes normally I found the same problem came back up; the brakes were starting to come on or locking up. I can move the truck in granny gear but it hard. Today after 12 hours I went out to the truck after it sat in the cold weather and found when I hit the brake pedal the brakes seems to feel like the locking had diminished or is gone. Yesterday after the episode when I last used the brake pedal it was tight.

My questions are:

1. When the brake lines leave the master cylinder they hit a block in the frame above the front wheel. Is this block some kind of valve ? I see a wire landed on the block too. I figure its a switch leg to indication that the brakes have failed. ( After 48+ years I know I have to rewire this circuit as most of the wiring to the dash is not functioning ).
2. How dose the electrical switch indicate circuit function ?

3.I also think I may have air in the lines that is heating up and expanding / thus applying the brakes as I described" after it warms up" but maybe the block that I described has a valve in the circuit that is sticking closed and if so the fluid is not returning to the master cylinder when I release the brake pedal.

Well send me comments and thanks for the time. Mum-a-mia..
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Re: Brakes Locking up ! 1969 W200, Help.

Postby MadMC63 » Fri Jan 13, 2017 9:55 am

On a 69 you should have a duel master cylinder. The front large reservoir is for the front brakes and the smaller rear is for the rear. I think it's a 60/40 pressure split at the proportioning valve on the frame and that is where the wire attaches. It's a pressure switch that turns on the dash brake light in low pressure situations. There is a right way and a wrong way to bleed the brakes. You need to start at the furthest point and work your way back to the closest. Right rear, left rear, right front and then left front. If you don't follow these simple steps you could end up with hydrolock where air is permanently trapped in the system creating a full, firm pedal. Since you already changed the master cylinder you need to insure it has been bleed correctly and completely. Most master cylinders come with a bleed kit if not you can use your fingers to cover the holes while bleeding. The best way to bleed the master cylinder is on a bench in a vise. You also need to insure your rubber lines aren't at fault. Rubber lines have a tendency to collapse internally restricting fluid flow in one direction or the other.
Before doing anything you need to determine where the problem is i.e. the front or rear axle. Single wheel or multiple wheels simultaneously. Then move forward from there.
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Re: Brakes Locking up ! 1969 W200, Help.

Postby PwrWgnDrvr » Fri Jan 13, 2017 11:47 am

The "block" on the frame is not a proportioning valve. It is not a valve of any sort. It is a safety switch only. Its purpose is to sense a lack of pressure in either the front or rear circuits and then illuminating the light on the dash to indicate this failure. It does not affect, or control brake operation in any way. This is all diagrammed in detail in the FSM.

Your problem is the rubber flex hoses at the front wheels and/or the one at the rear axle. The rubber deteriorates internally and they swell shut. U can push fluid thru them when u apply the brakes, but it returns to the master very slowly or not at all, thus "locking up" the brakes. Replace the hoses and your problem will be solved.
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Re: Brakes Locking up ! 1969 W200, Help.

Postby cometphoton » Fri Jan 13, 2017 4:21 pm

MadMC63 wrote:On a 69 you should have a duel master cylinder. The front large reservoir is for the front brakes and the smaller rear is for the rear. I think it's a 60/40 pressure split at the proportioning valve on the frame and that is where the wire attaches. It's a pressure switch that turns on the dash brake light in low pressure situations. There is a right way and a wrong way to bleed the brakes. You need to start at the furthest point and work your way back to the closest. Right rear, left rear, right front and then left front. If you don't follow these simple steps you could end up with hydrolock where air is permanently trapped in the system creating a full, firm pedal. Since you already changed the master cylinder you need to insure it has been bleed correctly and completely. Most master cylinders come with a bleed kit if not you can use your fingers to cover the holes while bleeding. The best way to bleed the master cylinder is on a bench in a vise. You also need to insure your rubber lines aren't at fault. Rubber lines have a tendency to collapse internally restricting fluid flow in one direction or the other.
Before doing anything you need to determine where the problem is i.e. the front or rear axle. Single wheel or multiple wheels simultaneously. Then move forward from there.


Thanks for the tips. I didn't do the initial brake work as it was completed when I made the purchase. I bet I have some kind of Hydro lock. I bet the mechanics didn't follow the process you described. I will remove the Master cylinder and bleed then bleed as you described. Will this eliminate the hydro block?
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Re: Brakes Locking up ! 1969 W200, Help.

Postby cometphoton » Fri Jan 13, 2017 4:24 pm

PwrWgnDrvr wrote:The "block" on the frame is not a proportioning valve. It is not a valve of any sort. It is a safety switch only. Its purpose is to sense a lack of pressure in either the front or rear circuits and then illuminating the light on the dash to indicate this failure. It does not affect, or control brake operation in any way. This is all diagrammed in detail in the FSM.

Your problem is the rubber flex hoses at the front wheels and/or the one at the rear axle. The rubber deteriorates internally and they swell shut. U can push fluid thru them when u apply the brakes, but it returns to the master very slowly or not at all, thus "locking up" the brakes. Replace the hoses and your problem will be solved.



All the hoses are new. They were changed before I purchased the truck.
Thanks for the details on the block and safety switch as I figured it was a proportioning valve.
Looks like its some kind of hydro lock.
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Re: Brakes Locking up ! 1969 W200, Help.

Postby cometphoton » Fri Jan 13, 2017 4:30 pm

PwrWgnDrvr wrote:The "block" on the frame is not a proportioning valve. It is not a valve of any sort. It is a safety switch only. Its purpose is to sense a lack of pressure in either the front or rear circuits and then illuminating the light on the dash to indicate this failure. It does not affect, or control brake operation in any way. This is all diagrammed in detail in the FSM.

Your problem is the rubber flex hoses at the front wheels and/or the one at the rear axle. The rubber deteriorates internally and they swell shut. U can push fluid thru them when u apply the brakes, but it returns to the master very slowly or not at all, thus "locking up" the brakes. Replace the hoses and your problem will be solved.


Can you post the FSM or send me a link.
Thanks
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Re: Brakes Locking up ! 1969 W200, Help.

Postby MadMC63 » Fri Jan 13, 2017 4:57 pm

He's exactly right about the Block on the frame. Sorry about that. I'm sure all you need is more bad info. You never stated how they lock up only that they did after using them. I could be wrong but I seem to recall a brake binding issue where the shoes weren't installed correctly. Short shoe in the front and long shoe in the rear.
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Re: Brakes Locking up ! 1969 W200, Help.

Postby Hobcobble » Fri Jan 13, 2017 5:38 pm

Adjust your push rod that is between the brake pedal and the
master cylinder piston. :idea :2cents Back if off slightly to reduce its overall length.
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Re: Brakes Locking up ! 1969 W200, Help.

Postby PwrWgnDrvr » Fri Jan 13, 2017 8:24 pm

Somebody else had this problem recently and it was debris stuck in the orifice where the lines attach to the master.
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Re: Brakes Locking up ! 1969 W200, Help.

Postby cometphoton » Sat Jan 14, 2017 7:46 am

PwrWgnDrvr wrote:Somebody else had this problem recently and it was debris stuck in the orifice where the lines attach to the master.


Thanks for the tip. I plan to remove the Master Cylinder so I will check this too.
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Re: Brakes Locking up ! 1969 W200, Help.

Postby cometphoton » Sat Jan 14, 2017 7:47 am

Hobcobble wrote:Adjust your push rod that is between the brake pedal and the
master cylinder piston. :idea :2cents Back if off slightly to reduce its overall length.
John


Planning to remove the master so I will adjust this too.
Thanks
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Re: Brakes Locking up ! 1969 W200, Help.

Postby sweatybetty » Sun Jan 29, 2017 10:33 pm

MadMC63 wrote:. The front large reservoir is for the front brakes and the smaller rear is for the rear.


this is incorrect. the cylinder closest to the firewall goes to the FRONT BRAKES. the cylinder toward the radiator goes to the REAR BRAKES
on some rebuilt master cylinders, there is a small rubber valve behind the fitting that the brake lines attach to. it is basically a one-way valve. it will let fluid out to the brakes fast, but will keep pressure in the lines for a little while. take those out (if they are there) and it should solve your problem
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Re: Brakes Locking up ! 1969 W200, Help.

Postby cometphoton » Sun Apr 16, 2017 12:25 pm

OMG....This was it....the adjustment was loose as a goose and probably tighten up.
I thru in the towel and purchased a new booster first.
When I was taking the old one out I did take note of the adjustments
As I installed the new booster the adjustment required a few turns; I assume this was the difference.
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