Mark
Posted a lot
Posts: 3,818
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Aug 30, 2015 12:47:52 GMT
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I have replaced the whole braking system on my E21 BMW and fitted a remote servo from Stafford Vehicle Components. This is the only one available for a dual circuit system.  Everything went together OK and is plumbed in as per instructions. The brakes bled through nicely and the pedal feels great. The problem is there is pressure building in the front circuit, to the point where the brakes lock on. This is easily cured by cracking the bleed nipple on the servo, but as soon as you start using the brakes again, the pressure starts to build. Just wondered if anyone might know what's going on here and what I can do to remedy the problem? Master cylinder is 'as new' but it feels as if there's a valve somewhere, or something similar, that's not allowing all the fluid to return as it should? Any help would be much appreciated. |
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Last Edit: Aug 30, 2015 12:48:36 GMT by Mark
BMW 320d (fridge on wheels)
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Aug 30, 2015 13:29:09 GMT
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Bias valve ?
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Aug 30, 2015 15:41:59 GMT
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Are you running a lot of piston area up front? I'm just wondering if perhaps they have dialed a little bit of residual pressure holding into it to stop pad knock off, which works fine with the normal stuff it's used with but is enough to make the brakes drag on your setup.
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Aug 30, 2015 16:40:56 GMT
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Hi, sounds like it might be an air valve problem. There's vacuum both sides of the diaphragm, when you put your foot on the brakes the hydraulic pressure moves the air valve and lets air into the back of the chamber and it then pushes the diaphragm into the vacuum and the pushrod pushes the piston giving you assistance. It appears to only affect the fronts because they are more effecient. Servo,s like that are usually recommended to be mounted with the air valve above the centreline, so try that first.
Colin
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Last Edit: Aug 30, 2015 16:43:57 GMT by colnerov
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Aug 30, 2015 19:15:41 GMT
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does it need a bit more clearance on the push rod? could maybe lash it up with a temp spacer to see if its any differant
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91 golf g60, 89 golf 16v , 88 polo breadvan
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Kieran
Posted a lot
Posts: 1,092
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Aug 30, 2015 19:20:15 GMT
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Is the master cylinder piston allowed to return fully?
Maybe the servo is stopping the piston returning.
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The Ashby Jackson fleet:-
1979 Mini Clubman 1.8 K series
1978 Skoda 110r Project 130RS K-oupe
1978 Austin Allegro 1500 SDL Estate
1984 BMW K100 Sidecar outfit
1999 Yamaha FZS 1000 Fazer
1991 Kawasaki ZXR400 race bike
2002 Kawasaki ZX9r race bike
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Midas
Part of things
Posts: 505
Club RR Member Number: 14
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Brakes - HELP!!Midas
@midas
Club Retro Rides Member 14
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I'll be interested to see how you get on with this, I'm looking to fit one of these to the Midas when I get it back on the road.
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MrSpeedy
East Midlands
www.vintagediesels.co.uk
Posts: 4,786 
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Sept 1, 2015 18:56:46 GMT
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Hi, sounds like it might be an air valve problem. There's vacuum both sides of the diaphragm, when you put your foot on the brakes the hydraulic pressure moves the air valve and lets air into the back of the chamber and it then pushes the diaphragm into the vacuum and the pushrod pushes the piston giving you assistance. It appears to only affect the fronts because they are more effecient. Servo,s like that are usually recommended to be mounted with the air valve above the centreline, so try that first. Colin This! I had a similar problem with the old girling remote servo on our 400e. We rebuilt it when repairing/restoring it earlier this year. The new seals were too stiff in the air valve piston, which stopped it returning, which meant the air valve was held open, which meant that the vacuum continued to apply braking effort with no pedal input. Hopefully you follow what I mean there?! It took a bit of tracking down the problem but once sorted the seals it was fine |
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Mark
Posted a lot
Posts: 3,818
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Thanks for all the replies chaps - much appreciated.  So in order to get the air valve above the centreline, do I need to mount the servo with that end of the servo physically higher? (the end that needs the bleed nipple cracked to release the build up of pressure) As you look at the picture above, it's the furthest end (nearest firewall) that's having the issue. Can I just swivel the mounting bracket and drop the nearer end down a couple of inches, or does it need to be much more that that? Thanks in advance. |
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BMW 320d (fridge on wheels)
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Hi, you need to rotate it along its length so it's like the pic on the website. -  It could still be the air valve but see if this improves matters first. Colin |
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Last Edit: Sept 2, 2015 7:57:55 GMT by colnerov
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Mark
Posted a lot
Posts: 3,818
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Ahhhhh, yes I see what you mean now.
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BMW 320d (fridge on wheels)
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Mark
Posted a lot
Posts: 3,818
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Well we adjusted the push rod on the master cylinder and replaced the one way valve in the rubber servo hose. As a result the brakes no longer lock on! I still need to give the pedal a healthy shove though, so unsure the servo is giving as much assistance as I might like? I'm also wondering if a larger / smaller bore master cylinder might help? |
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BMW 320d (fridge on wheels)
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Sept 3, 2015 14:24:52 GMT
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a m/c thats too big will give a wooden feel to the pedal , couple of mm is all it takes to ruin the feel
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91 golf g60, 89 golf 16v , 88 polo breadvan
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Mark
Posted a lot
Posts: 3,818
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Sept 3, 2015 14:43:54 GMT
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I think I'll drop back to the OE and replace the E12 one I'm running.
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BMW 320d (fridge on wheels)
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Sept 3, 2015 15:38:30 GMT
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what size pistons are you running compared to original?
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