Classic cars forum & vehicle restoration.

[Clactonguy]

rover p6 '74 auto . having replaced I think entire braking system including all four callipers. pipes. etc after an event at a car meet where my front brakes stuck on! had to release pressure in hydraulic line. to get pads. to stop gripping . slight sponginess but brakes were ok.
now with exchange servo fitted went for MOT ( passed) and on way home. front brakes decided to stick on again. full power form a v8 managed a fine 15 mph though. as pulled into drive ( approx 3miles) smoke from over heating brakes wasn't pretty . both hubs hot.checked rear hubs .one warm ( not hot) and other. cool ??
issue now being what is happening and how to fix. not funny and. can't risk driving when brakes can stick again without warning.
anybody have any ideas or same issue?

[roverdriver]

Firstly, your rear brakes are a long way from the rear hubs, as the discs are mounted adjacent to the diff. A warm hub suggests a bearing getting warm.

I would suspect the rubber hoses to the fronts as a first check. They can break down inside and act as a valve so stopping the release of pressure back to the master cylinder.
_________________
Dane- roverdriver but not a Viking.

[Clactonguy]

have replaced all brake pipes .including new braided flexi. good point the Rea brakes mounted inboard though no smoke was noted from under rear end and enough smoke to send smoke signals to. red Indians at front brakes!
would have though if all four brakes were. 'on' even the 3.5l engine would not have been able to propel car at any speed.
so at moment still looking at ideas why front sticking on as servo unit and air valve assembly plus master brake cylinder etc all replaced previously.
didn't think to jack car up before releasing hydraulic pressure. and check if rear wheels were in fact 'free' to rotate .
wondering if there is a pressure differential unit in brake line ( most cars have these to stop excess pressure to rear brakes)
if so could that be an issue ?

[alanb]

I doubt the rear bias valve would be the problem as this restricts the pressure to the rear wheels rather than increase pressure to the front wheels, if it were the rear brakes giving the problem then it would be worth looking into.
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old tourer


Morris 8 two seater

[MVPeters]

[Clactonguy]

yes. master is adjustable but was set correctly according to rover setting instructions. would have thought both front and rear would have been held on if it was failing to release. pedal stays in same position . brakes are not 'rubbing' but fairly hard on. correct I didn't check rears as. under car so hubs. stay. cool .however smoke only coming from front 2 hubs.discs when stopping eventually. car proceeds under almost full power at about 15mph!
a single hydraulic. circuit so at moment. looking at air valve being a possible issue though this was. refurbished unit with servo and has done around 250miles since being fitted.
bit of a head scratcher ..( ouch..splinters ) can't see at moment how rear brakes are separate from front if on same circuit? so may. well have had rears. rubbing but with no smoke from them seems odd as front 2 def cooked well.next door. popped over as I drove in as seeing the smoke from front. also left a trail of coolant as. engine started to overheat showing how hard held on.

[lowdrag]

Strangely, I suffer from the same problem and never know when it will occur. Sunday we were out for a run and the front N/S locked on. Again, pull over, release pressure, and for the rest of the day nary a problem. I might go months before it reoccurs, and since most of the braking system was renewed three years ago it is puzzling. Never any other brake - just the front near side.

[Farmer John]

Hi, several things to check.
Make sure that you do not have a "line pressure" cylinder. They are for drum brakes and have a wee spring loaded valve in the cylinder right at the end where the brake pipe attaches.
Wheel bearing adjustment is important, the pads need to be "knocked off" by movement of the disc, there have been hubs built with runout to achieve the same end yet another maker require three thou end float at the hub. Unlikely your problem, important for proper operation.
Whatever else has been established you simply must make sure that there is freeplay between the pushrod and the master cylinder piston. This includes checking that none of the parts is binding and eliminating freeplay after you have checked it. There should be a visible "pulse" of fluid in the reservoir just as the pedal starts to move before the port is blocked off and fluid sent to the brakes. Beware! The pulse varies from squirt you in the eye to a barely discernable disturbance of the fluid surface.
It is possible to lubricate the piston and o-ring so well that the o-ring does not "roll" on application but allows the piston to slide through the o-ring. The o-ring should roll forward and then roll back thus pulling the piston away from the disc.
Jack one wheel, remove the wheel, stamp on the brake. You should be able to turn the hub without the wheel on but maybe with great difficulty.
Do not remember but these are not floating calipers are they?

John

[Bitumen Boy]

[ukdave2002]

There are some reproduction Girling m/c's that didn't release the fluid back correctly, my Bedford CA had one fitted and if driving in stop start traffic the brakes would begin to bind.

Fitting a puka Girling cylinder cured the problem

Dave

[lowdrag]

I've had the master cylinder for years, and we rebuilt it with new seals etc. I guess I'll flush the system and start again, then suck it and see. Up-down; up-down.............