I'm nearing the end of a restoration on a 1974 v8 auto. A test drive revealed that the brakes aren't quite right. During the restoration it's had 4 x recon calipers, 4 x new discs, new pads all round, a new master cylinder and a refurb kit for the servo unit. The brakes work very well in that they stop the car superbly but the pressure required to feel the brakes acting seems to vary from a very light tap on the pedal to 15mm or so of pedal movement. I also can't tell what I'm going to get. Sometimes it feels to give full servo assistance and other times, very little, sometimes they grab on instantly.0, sometimes they need a good shove! They stop the car well but I don't know what I'm going to get when I press the pedal. In an attemp to remedy this today, I've removed the master cylinder just to check that it was a new one that I fitted (couldn't remember if I'd put the new one on my other car), and it is. I also checked the plunger action us nice and smooth and all is ok. The pipe running from master to servo had quite a tight bend in it so I straightened it up, refitted the master and bled the system. It seems slightly better but still not quite right. To test the servo I depressed the pedal several times, started the engine with my foot on the brake and expected the pedal to drop slightly, indicating a healthy servo. It did the opposite - the pedal rose! Have I got something back to front? There is a healthy vacuum from the inlet manifold and the 2 vacuum pipes are in good nick. Any thoughts?
Scratching my head on brakes
- Thread starter badrover
- Start date
Thanks again for the advice. By 2 x vacuum hose I mean the main one from the inlet manifold and the one that links from the servo casing to the air control valve. The spring modification under the air control valve is missing too. I have one to put in though. I guess that this on its own won't be my problem though.
If you removed just the air control valve, then no, you won't need to bleed the brakes again.
Ron.
Ron.
Yep that's fine. Have you fitted the modification spring that was not in there originally?
Ron.
Ron.
I refurbished the servo a couple of years ago, ready for fitting. When Putting it all back together it I must have referred to a manual that does not picture the spring so I didn't include it. I know that I have a spare spring in my box of bits and bobs and I wondered where it came from! It's only doing research tonight that I've found out about the spring modification so it will be the first thing to be done in the morning. In short, the servo had the spring fitted before my refurbishment and I didn't put it back! Lesson learnt - take lots of photos! I hope this solves my problem but I reckon there's more to it.
badrover wrote,...
That could be part of the problem. When you rebuilt it, did you use any form of rubber grease or only brake fluid? Some types of rubber grease are quite viscous, and when used on the air control piston, can inhibit correct operation.
Ron.
That could be part of the problem. When you rebuilt it, did you use any form of rubber grease or only brake fluid? Some types of rubber grease are quite viscous, and when used on the air control piston, can inhibit correct operation.
Ron.
I think you may be right about the piston being suspect after reading these 2 threads. The car is at my parents garage or I would go out and check now! Will have a good look at the piston in the morning. I can't remember re lubrication but I guess you are suggesting just brake fluid for the piston?
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I had a good look at the piston today and the bore contained some rusty looking fluid. I cleaned all this out and poked some welding wire down the hole at the bottom which seemed a little blocked. I used some wet and dry, lightly on the bore and piston and fitted a new piston seal. Then I pushed the pedal a little to clear and residual muck with clean fluid and put the piston back in. I also refitted the spring and can confirm that they feel better. I've not been for a run, just back and forth a little and they seem far more predictable. Disappointingly the pedal is still pushing up, rather than dropping after pumping as described earlier. I will go for a run over the next few days to get a better idea.
badrover wrote,...
I have had that unpleasant experience before. Most disconcerting when applying the brakes so as to wash off speed, only to have the pedal push back at you.
In my case, the booster in question had been in service for many years, and was in need of an overhaul. For that to occur, the brake fluid which runs between the booster slave cylinder and the master cylinder would need to be under pressure, above the normal master cylinder working pressure. In addition, the resultant flow will be in the opposite direction, thereby pushing the piston within the master cylinder back up the bore, hence the pedal lifts rather than falls, as would normally be expected.
I can only imagine therefore that one or more of the seals within the slave cylinder is not functioning as it should be.
This is going to be a pain, but I think that it would be fortuitous to dismantle the slave cylinder, check the condition of the bore and rebuild it with another seal kit. If it has not been re sleeved in stainless, then now would be an opportune time for that enhancement to occur.
Ron.
I have had that unpleasant experience before. Most disconcerting when applying the brakes so as to wash off speed, only to have the pedal push back at you.
In my case, the booster in question had been in service for many years, and was in need of an overhaul. For that to occur, the brake fluid which runs between the booster slave cylinder and the master cylinder would need to be under pressure, above the normal master cylinder working pressure. In addition, the resultant flow will be in the opposite direction, thereby pushing the piston within the master cylinder back up the bore, hence the pedal lifts rather than falls, as would normally be expected.
I can only imagine therefore that one or more of the seals within the slave cylinder is not functioning as it should be.
This is going to be a pain, but I think that it would be fortuitous to dismantle the slave cylinder, check the condition of the bore and rebuild it with another seal kit. If it has not been re sleeved in stainless, then now would be an opportune time for that enhancement to occur.
Ron.