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Hydrovac Brakes
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Rootes75



Joined: 30 Apr 2013
Posts: 2728
Location: The Somerset Levels

PostPosted: Fri Jun 01, 2018 7:41 pm    Post subject: Hydrovac Brakes Reply with quote

I have got my Commer lorry running nicely now but I noticed today that with the engine running when the brake pedal is depressed it fails to return to its starting position.
The system is hydrovac, the air for the hydrovac unit is taken from the manifold so should build up when the engine is running.
When the engine is off the pedal can be depressed and it will return to its starting position, it just doesn't when the engine is running.
There is plenty of brake fluid in the system and there are no leaks.

Anyone any ideas?

I did wonder if during the rebuild of the hydrovac the leather diaphram did not seal correctly and air is not being contained?
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1937 Ford 7w
1937 Hillman 80
1946 ERF C.I.5
1947 Hillman Minx
1955 Hillman Minx Mk8
1950 Commer R541
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Penman



Joined: 23 Nov 2007
Posts: 4080
Location: Lancashire

PostPosted: Sat Jun 02, 2018 11:45 am    Post subject: Reply with quote

Hi
I posted a link to your question on STC61, thinking there might be someone on there with experience of them.
There is now an answer, and might yet be more to come.
http://www.sct61.org.uk/msgboard.htm
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Bristols should always come in pairs.

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Rootes75



Joined: 30 Apr 2013
Posts: 2728
Location: The Somerset Levels

PostPosted: Sat Jun 02, 2018 6:40 pm    Post subject: Reply with quote

Thank you for that Penman. I am going to start her again in the morning and check properly for air leaks on the pipework to the hydrovac.
_________________
1937 Ford 7w
1937 Hillman 80
1946 ERF C.I.5
1947 Hillman Minx
1955 Hillman Minx Mk8
1950 Commer R541
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Rootes75



Joined: 30 Apr 2013
Posts: 2728
Location: The Somerset Levels

PostPosted: Sat Jun 02, 2018 6:40 pm    Post subject: Reply with quote

Thank you for that Penman. I am going to start her again in the morning and check properly for air leaks on the pipework to the hydrovac.
_________________
1937 Ford 7w
1937 Hillman 80
1946 ERF C.I.5
1947 Hillman Minx
1955 Hillman Minx Mk8
1950 Commer R541
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Rootes75



Joined: 30 Apr 2013
Posts: 2728
Location: The Somerset Levels

PostPosted: Sun Jun 03, 2018 1:44 pm    Post subject: Reply with quote

An update, I have been down the lorry this morning and checked for any air leaks, none are audible.
I have re-tightened all fittings on the system. The one thing of note is that on tickover the brake pedal when depressed stays down, if you then give it some throttle the pedal rises to starting position. So obviously is getting more air.
Perhaps on tickover it doesn't produce enough air to fill the hydrovac chamber?
_________________
1937 Ford 7w
1937 Hillman 80
1946 ERF C.I.5
1947 Hillman Minx
1955 Hillman Minx Mk8
1950 Commer R541
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Rootes75



Joined: 30 Apr 2013
Posts: 2728
Location: The Somerset Levels

PostPosted: Sun Jun 17, 2018 5:55 pm    Post subject: Reply with quote

Well, after much investigation the Hydrovac works correctly. We have found the master cylinder on the hydrovac is at fault.
We thought about it and realised the hydrovac overcomes the resistance of the hydraulic pressure. So if there is no pressure on the pedal there is no hydraulic pressure? First of all we went to bleed the back brakes and found no fluid coming through at all. We checked the resevoir and the fluid was coming out of that and the inlet pipe on the master cylinder but nothing was then coming out of the master cylinder to the brake pipework.
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1937 Ford 7w
1937 Hillman 80
1946 ERF C.I.5
1947 Hillman Minx
1955 Hillman Minx Mk8
1950 Commer R541
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jp928



Joined: 07 Jun 2016
Posts: 247
Location: Australia

PostPosted: Sat Jun 30, 2018 2:23 am    Post subject: Reply with quote

Good that issue is sorted, but seeing as its a 'hydrovac', it operates based on low pressure (vacuum) on one side of the diaphragm/piston, not on a volume of air , but on the absence or reduced mass of air. At idle (throttle plate nearly closed) the vacuum will be at its lowest, best, pressure.
jp 26 Rover 9
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Rootes75



Joined: 30 Apr 2013
Posts: 2728
Location: The Somerset Levels

PostPosted: Sat Jun 30, 2018 8:24 am    Post subject: Reply with quote

The master cylinder is now rebuilt and back on the hydrovac, all pipework is fitted up tight so today we will top up the fluid and see if we can get it all set up to work.
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1937 Ford 7w
1937 Hillman 80
1946 ERF C.I.5
1947 Hillman Minx
1955 Hillman Minx Mk8
1950 Commer R541
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Penman



Joined: 23 Nov 2007
Posts: 4080
Location: Lancashire

PostPosted: Sat Jun 30, 2018 9:38 am    Post subject: Reply with quote

Hi jp

I think all car power assisted braking works like that, but you spelling out has got me thinking.

Come off the gas, vacuum increases, start braking and it has most assistance; but what happens if you are heel & toeing?
The application of gas would reduce the braking effect.
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Bristols should always come in pairs.

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Straight 6
V8 V10
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jp928



Joined: 07 Jun 2016
Posts: 247
Location: Australia

PostPosted: Sat Jun 30, 2018 10:02 am    Post subject: Reply with quote

Correct, except in cars with vacuum tanks - in the 50s, 60s, 70s, Jag, Rover, RR, MB etc. To assess how many brake application you can get of the system, turn off the engine and see hany full brake applications you get before the pedal goes hard, and the sound of air going in ceases.
In fact drum braked RR had a very nice booster until quite late. There was a right angle drive off the back of the gearbox driving a clutch plate about 6" dia; it had a small shaft with a coarse grooved thread on it; the brake pedal, as well as pushing on the master cylinder also pulled a disc into engagement with this clutch plate, and its rotation helped to push the brake push rod. When I worked in a brake workshop I was shown how to adjust this, and it was quite easy and intuitive . The beauty of the system was that if you locked up the rear wheels, the boost came off until the wheels rotated again, and so on .
jp 26 Rover 9
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Rootes75



Joined: 30 Apr 2013
Posts: 2728
Location: The Somerset Levels

PostPosted: Sat Jul 21, 2018 2:35 pm    Post subject: Reply with quote

Well, patience pays off. All refitted again and with the rear passenger cylinder bleeder loose I pumped the pedal for 20 mins. This time though fluid made its way through, and a good amount too. I loosened off the drivers side rear cylinder bleeder and the same there too.
So now we know fluid is being pumped through, we can start bleeding the system correctly.
_________________
1937 Ford 7w
1937 Hillman 80
1946 ERF C.I.5
1947 Hillman Minx
1955 Hillman Minx Mk8
1950 Commer R541
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View user's profile Send private message
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