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  #1  
Old 24-09-18, 04:49
Malcolm Towrie Malcolm Towrie is offline
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Default Ferocious brakes on our M8 Greyhound

We had a problem with the brakes binding on our M8. I found the wheel cylinder bores corroded enough to restrict piston movement so the hydraulic pressure would push the pistons out but the much lower spring return force couldn't pull them back. Honing the bores and greasing the pistons with "red rubber" grease fixed the problem. With the Hydrovac booster and the twin cylinders acting on each of six wheels, the brakes are phenomal, so much so they can be a bit ferocious. A light touch is needed on the pedal. Drivers here are not used to using a light touch on WWII vehicles,so there are complaints!
Was this normal? I'm not too familiar with the Hydrovac unit. Can they fail in a way that causes this?
I have seen contaminated brake shoes cause this problem but the shoes were all found clean and dry. And all the shoes were adjusted out to give a very light rub on the drum.

Malcolm
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  #2  
Old 24-09-18, 05:36
rob love rob love is offline
carrier mech
 
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I had the same a couple years ago on the old GMC M135 at work. Another unit had done the brakes previously, and the slightest touch (anything more than two toes worth of pressure) would just about send you through the windshield. There was also the feeling that they were sticking. The wheel cylinders were about 8 years old at that point, and I did find that one or two of them were starting to leak. As well, whomever did the brake cleanup used a disc sander to take the ridges off the inside edge of the drums, leaving about an inch of drum in poor shape. Lastly, they had used the master cylinder off an M35 MLVW, which, while it looks the same as an M135, has a small residual check valve inside.



Rather than trying to repair faulty work of others, I went clean with 12 new wheel cylinder, 6 new drums, and new shoes all around. I also pulled apart the master cylinder and removed the check valve. Part of my motivation to put all new was also to take advantage of the parts still being available in the supply system for the 1-1/2 ton trailers.



Long story short, on completion the brakes worked out great. They feel exactly as they should, and hopefully will remain trouble free for another decade, at which time they will likely be the problem of someone else. The truck also had an airpac from an M35 installed.....I left that alone.



So which exact cause was resulting in the brakes locking on I cannot say. All I can say is that by doing it by the book (or as close as one can considering what parts are available), the problems disappeared.
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Old 24-09-18, 13:10
David Herbert David Herbert is offline
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This has got to be a problem in the Hydrovac unit. It would seem not to be moderating the boost to the pedal pressure so it is giving either full boost or nothing. I would look for siezed parts within the Hydrovac.

David
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Old 24-09-18, 20:52
marco marco is offline
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Does it have the (original) dual line Hydrovac?
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Old 25-09-18, 04:13
Malcolm Towrie Malcolm Towrie is offline
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Based on your suggestions, I think I need to put a 2000 psi brake pressure gauge on a wheel cylinder and press down progressively on the pedal with the engine running. If I see sudden or erratic changes in pressure, the Hydrovac unit is acting up. If pressure ramps up smoothly, it's likely the shoes, as drums are in excellent condition.
This M8 is in great condition but it rarely gets driven. Maybe the first thing I should do is thrash it round the track for half an hour with lots of braking to see if some bedding in helps.

As usual, thanks for making me think outside the box.

Marco, I'm not sure what you mean by dual line. It has one line coming in from the master cylinder and one line coming out for all 6 wheels. This is the old style cylindrical hydrovac that was used from the 40s to the 60s.

Malcolm
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Old 25-09-18, 05:01
lynx42 lynx42 is offline
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Is the vacuum line coming from the right place? I don't know about an M8 but I had a Dodge for sale a few years ago and a prospective buyer took it to his mechanic to give it the once over. When it was returned he didn't say anything but that he would think about it. The next day another prospective purchaser took it for a test drive with me and nearly put me through the windscreen. Turned out the first blokes mechanic had moved the vacuum line to the centre of the intake manifold away from where it should have been at no.6 cylinder and there was far too much vacuum . The mongrel first bloke came back to me with an low offer "as there was a big problem with the brakes". He got sent packing. Youi might say I wasn't very happy with him.
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  #7  
Old 25-09-18, 20:30
marco marco is offline
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Hello Malcolm,

See attached diagram showing the brake system on the M20/M8 armored car.
There are two hydraulic lines running from the brake master cylinder to the Hydrovac.
The extra hydraulic line actuates a valve inside the Hydrovac controlling the brake action on partial application of the brake pedal. The absence of this extra line might be the cause of the "all or nothing" attitude of the brake system or it might indicate that at some time a different Hydrovac has been installed.

Years ago we restored a M20 armored car and fitted a Halftrack Hydrovac (single line) because an original one couldn't be found.
This Hydrovac has a higher outlet pressure compared to the original M20 example.
The result was that the brake application was very similar to what you experienced.
On full brake application the car could be stopped with all 6 tyres smoking..
I have more information on Hydrovacs, i'll go through it this week to see if there is usefull information to share.

Marco
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  #8  
Old 25-09-18, 22:41
David Herbert David Herbert is offline
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Could there be early and late M8/M20 instalations with different Hydrovacs ?

David
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  #9  
Old 26-09-18, 03:31
Malcolm Towrie Malcolm Towrie is offline
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Marco, I'll need to look at the layout on our M8 more closely, but I don't think that schematic is what we have so you may be onto something.
Your schematic is in TM 9-1743 but I noticed in that manual the brake drum has one of the wheel bearings incorporated in it, while our M8 does not. After pulling a wheel, you remove three slot head screws and the drum just pulls off the axle flange. So was a major brake redesign done sometime? Anyway, I'll look more closely at ours to confirm what we have.

Malcolm
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  #10  
Old 26-09-18, 06:24
Big D Big D is offline
Darryl
 
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Default Brakes

Hi all,

I think there were three variants of the hydrovac on the M8/M20. Mine is the long one that looks like the one used on the halftrack. There is a shorter stouter version (I think the latest?) that looks a little like the one on the White Scout Car. I’ll see if I can track down the article that describes the development/variations on these.
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  #11  
Old 27-09-18, 02:39
Malcolm Towrie Malcolm Towrie is offline
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Marco, I was wrong. What we have is exactly as per your schematic, with two lines coming back from the master cylinder to the Hydrovac. So we have the stock master cylinder and Hydrovac. As I said, I'll drive it some more and if it doesn't settle down, I'll check braking pressure.

Malcolm
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