Ferocious brakes on our M8 Greyhound

[lynx42]

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.

[marco]

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

[David Herbert]

Could there be early and late M8/M20 instalations with different Hydrovacs ?

David

[Malcolm Towrie]

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

[Big D]

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.

[Malcolm Towrie]

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