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Brake problems

3K views 16 replies 5 participants last post by  50LtCapri 
#1 ·
Need some help, I've got an XY wagon i'm just about to get on the road. It's got new booster, master cylinder, brake lines, running XF disc brakes(front), drum brake rear with new wheel cylinders, but i've driven 4-wheel drums that have better stopping power. there is no air in the lines. Any ideas on what the problem may be?
 
#8 ·
I assume it is a dual circuit master? If you stand on pedal with engine off, how far does it go? If you retain pressure and start engine, how much further does it go? It is starting to sound like air in rear circuit.
 
#10 ·
Normally, boosted travel will be more than unboosted travel. Could mean low vacuum or not very effective booster. Could also mean that there may be some air in the rear circuit. Can you feel the front and rear sections of the master working in the pedal travel? Remember, the front circuit only operates when the rear circuit has enough back pressure to push the secondary piston in. Air in rears will completely stuff up the operation of the fronts.
 
#11 ·
Well i'm running a stock 250 2V, so there should be plently of vacuum. The booster was rebuilt to suit the brakes, and the brakes have been bled about 10 times now, theres no bubbles in the master cylinder when the brakes are applied, so i don't think theres any air in the lines. could the back brakes being adjusted up too much cause not enough pressure for the front brakes?
 
#13 ·
If the rears were way out of adjustment, then the slaves would require excessive fluid to obtain braking and build back pressure to operate the fronts. You would have to clear the crack valve setting first though, so the pedal would behave normally and then drop to operate more brake. Being adjusted way up shouldn't cause any dramas. Bubbles in the master is not a guaranteed indicator. Have you tried cracking the line to the rear at the master and bleeding it there?
 
#15 ·
Sometimes with secondhand disc brake calipers the slide pins that hold the caliper onto the ub can be rusted , therefore the brakes work, but really badly. This is because when the cylinder side pad pushes against the disc, the other side of the caliper won't get pulled up onto the disc, so you in effect are trying to stop the car on one side of the disc on both calipers, instead of squeezing the disc from both sides. Get someone to apply the brakes while ytou are under the car and see if the WHOLE caliper slides away from the disc when they are applied. If they don't, then theres your problem, if they do, then, sorry, it's something else.
 
#16 ·
I'm starting to think it might be something to do with the pedal to booster pushrod. I took the master cylinder off last night and found that the rod from the booster to master cylinder wasn't moving at all when the pedal was applied. I adjusted the pedal-booster rod and got movement. I'm about to bleed the brakes and see if it made any difference.
 
#17 ·
What size master cyclinder?

XF's are 1" where as XY with 250 would be smaller.

Get a master cyclinder from a XY V8.
 
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