I've finally finished fitting my obp pedal box, fitted the bias bar and bled the brakes. I took it for a quick drive yesterday and the brakes
didn't seem to work very well. Today I went back with a fresh head and adjusted the threads on the cylinders so they both only go in the clevis
by about 15mm. Took it for another drive and this time the brakes worked very well and for the first time since owning it the wheels locked up (only
front left though)
What's the best setup and which wheels should lock up first, front or rear?
Locking the rears up is bad, the fronts must lock first.
Front
Thanks so does that mean when looking at the push rods the rear one will be longer as more of the front ones screwed in the clevis. Also what's the reason for the right front locking but not front left
http://www.obpltd.com/wordpress/?p=191
Worth a read..
Many thanks that's helped and answered the question that front push rod should be 5mm longer than rear but what about the full length of push rod, or is that just to determine how far forward or back you won't the pedal itself?
quote:
Originally posted by Sierra
Many thanks that's helped and answered the question that front push rod should be 5mm longer than rear but what about the full length of push rod, or is that just to determine how far forward or back you won't the pedal itself?
Sorry I'm new to this and just got a bit confused. By coming back do you mean when the pedal returns back when released because surely it will
always fully return as the thread is on the clevis end or do you mean when pedal is pushed that the push rod goes all the way into the cylinder.
The first time I fully threaded both push rods resulting in a very soft pedal that hit the bulkhead and didn't brake where as now only threaded
abit into clevis pedal is stiffer and brakes
quote:
Originally posted by britishtrident
Here be dragons !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Very very important the master cylinder pistons are allowed by the pushrods to come back fully against the stop washers at the end of the cylinder, if they don't the brakes will bind when the brake temperature increases. After a mile or so they will so solidly the car will come to a halt.
I don't have a stop on my brake pedal so surely it'll just come forward and stop when push rods are fully out. I've got the obp cylinders with the rubber boot over the pushrods
The master cylinders have a recuperating port (on some types a recuperating valve) that links the fluid in the cylinder bore to the reservoir
allowing top up fluid into the master cylinder bore and excess fluid to vent back to the reservoir. The recuperating port is only open when the
master cylinder piston is fully back in its' rest position hard against the stop washer. If the piston is even 1mm from stop it can close the
port.
When the brakes are used the fluid temperature in the calliper increases and the fluid expands. Unless the piston in the master cylinder can come
fully back to its rest position hard against the stop washer the fluid cannot return from the cylinder bore back to the reservoir. As a result if
the expansion of the fluid isn't vented back to the reservoir the pressure in the system increases and the brakes start to drag causing more
expansion and more drag from the brakes.
A lot of Locosters builders have had this problem and had their car grind to a halt with the brakes locked solid the first time the brakes were
used in anger, one builder was so convinced it wasn't the brakes but thought it was the rear axle seizing and changed the complete axle not
once but twice.
Jacking out the push rods on one or both master cylinders should only be done with great care to check the master cylinder pistons return fully
home, the pedal should also have a return spring.
If one circuit has excess pedal it should be tracked down, usually on cars with Sierra rear discs it is either air trapped in the rear calliper or
the self-adjuster for the handbrake not operating.
Brake bias bars only work best if the balance point adjustment puts the balance bearing close to the centre position, if when adjusted the
balance point is skewed to one side then ideally this should be fixed by changing the bore of one of the master cylinders.
Thanks for explaining everything. I think I'm going to double check the the pushrods do fully return before anything. Might also look into fitting a spring to clutch and brake pedal but for now they return fine