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Hello, this is my first discussion. I have previously posted on discussion "1968 Galaxie XL Power Brake Booster and Master Cylinder."

I have a 1968 Ford Galaxie XL convertible with front disc brakes and have a brake issue. I have restored the power booster, rebuilt the pressure differential and control valve, bought new proportioning valve, and no leaks in lines... but still has a low brake. any suggestions would be much appreciated...Mechanic is at a lose.

PDM
 

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Hello, this is my first discussion. I have previously posted on discussion "1968 Galaxie XL Power Brake Booster and Master Cylinder."

I have a 1968 Ford Galaxie XL convertible with front disc brakes and have a brake issue. I have restored the power booster, rebuilt the pressure differential and control valve, bought new proportioning valve, and no leaks in lines... but still has a low brake. any suggestions would be much appreciated...Mechanic is at a lose.

PDM
Hello PDM,

Your situation could be as simple as 1.) push rod in the booster to master piston needs adjustment and or 2.) Your master is missing or has a defunct residual pressure valve.

It also could be the rear shoes aren't adjusted, or you have a mismatch of parts if the booster has been replaced with another kind.

I recently did a large writeup on the entire brake system overhaul in my '68 XL. I cover these topics and more if you're interested here 1968 Disc Brake System Overhaul

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Discussion Starter · #3 ·
Hello PDM,

Your situation could be as simple as 1.) push rod in the booster to master piston needs adjustment and or 2.) Your master is missing or has a defunct residual pressure valve.

It also could be the rear shoes aren't adjusted, or you have a mismatch of parts if the booster has been replaced with another kind.

I recently did a large writeup on the entire brake system overhaul in my '68 XL. I cover these topics and more if you're interested here 1968 Disc Brake System Overhaul

Cheers
Much Thx DesertXL

Since I sent the booster out to be restored, we assume the push rod is adjusted right, but we agreed to recheck. The master came attached to the booster so assumed that ok too, I guess recheck that too. Rear shoes are good and all original part to my knowledge. Part #s match up to car after researching.

Fyi, when lines out master are blocked off pedal is hard.

Car has 1/2 pedal with pressure then after 3 stops red brake light comes on and proportioning valve thumps and shuts system down.

After reading you overhaul, do you recommend using the GM Disc/Disc combination valve in my case?

Thanks again,
PDM
 

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If you are going to replace the valve, I would suggest an adjustable one.
Are the front disc brakes factory or a modification?

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Very nice. Not a common option for full sized Ford/Mercury. And the first year for single piston calipers.

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Much Thx DesertXL

Since I sent the booster out to be restored, we assume the push rod is adjusted right, but we agreed to recheck. The master came attached to the booster so assumed that ok too, I guess recheck that too. Rear shoes are good and all original part to my knowledge. Part #s match up to car after researching.

Fyi, when lines out master are blocked off pedal is hard.

Car has 1/2 pedal with pressure then after 3 stops red brake light comes on and proportioning valve thumps and shuts system down.

After reading you overhaul, do you recommend using the GM Disc/Disc combination valve in my case?

Thanks again,
PDM
Hello PDM,

OK so you've listed some crucial information here. If you've blocked off both ports on the master and have a firm pedal right at the top you can forgo the booster, booster pushrod and master cylinder pistons.

From your statement of 3 stops the shuttle valve shifts and the hydraulic failure warning lamp comes on, this sounds like either

1.) The RPV is missing and or bad, plus the shoes aren't adjusted close to the drum. Either of these scenarios usually is not enough to trip the shuttle valve, but both would be.

I illustrated how to check for an RPV in my post. So that's something that needs to be done, if it's knackered or missing, you'll have to add an external one or try another new master that has one built in. The rear shoes adjustment is simply adjusting the star adjuster on each side till the shoes are in light contact with the drum. Also the drum needs to be true and smooth on the inside, if not have machined or replaced.

2.) The valving (shuttle and or rear proportioning) are compromised and leaking. As I mentioned in my post the proportioning valve uses specially moulded seals that are unobtanium. Unless it's a show car, do not try to use any of it. I would replace it with whatever you feel comfortable with. Me personally I like the reproduction GM Disc Disc combination valve. Just make sure you get the connector (most times it's sold separately). But it could be shrunk/hard seals pulling in air for the rear or just plain leaking. Even the seals in the shuttle valve can do this. Or it's making it impossible to properly bleed the rear brakes.

As a side note, it's not necessary to use an adjustable proportioning valve. The factory does a really good job of balancing the front and back gross brake bias. Now if you have the wrong inner diameter rear wheel cylinders (there are different sizes for a rough bias adjustment) or have wider tyres on the rear or have different size factory shoes or changed the weight on the car, then an adjustable rear proportioning valve may be in order. But honestly I wouldn't bother as cheap ones are going to be problematic, and the really expensive ones are still another failure or leak point with the adjustability, plus in my opinion they are unsightly.

Hope that helps.

Cheers
 

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I put out the suggestion of an adjustable valve without first asking if the system was stock or modified.
Mostly because stock disc brakes on a 68 full sized are not a common option. (I made an assumption about your brakes not always a good thing) By the time fourth Gen Galaxie/LTD came to be, single piston disc brakes came to be a standard equipment. (Mostly because the woes of the Kelsey/Hayes 4 piston caliper of the mid-1960s were no longer in service. And single piston worked as good as designed). ------
Then I asked the question as to if stock or modified. I should have asked this first.

The stock valve on the stock disc/drum system should work well.

The issue I have experienced on a 50 year old stock valves is the valve sticking. Typically this occurs in systems where the brake fluid is very old. The fluid does not last forever. Once the fluid aborbs enough moisture bad things happen. Debris gets into the tight spaces of the valve and during a bleeding routine the valve sticks and never moves back.

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Line Font Parallel Schematic Auto part
 

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Discussion Starter · #10 ·
Gentlemen

I got to say, you guys are awesome for sharing this info. Have struggled mightily this past year on the brakes, hopefully this is the solution.
Maybe I should have been more clear also, the caliper are not stock, but everything else I have not changed and looks stock.

The cars RPV is new, so if it is bad its a defect. Mechanic has checked the back drum brakes a few times and is sure good.

The pressure differential and control valve has been rebuilt, but the only answer we have right now is that it is worn inside and not functioning properly.

The GM valve sounds very tempting to try.

This is my pressure differential valve.
Brown Wood Rectangle Font Wall


As always, much thanks.

PDM
 
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