Mopar Muscle Association UK

Just changed my master cylinder out.
Devil of a job to get it to bleed properly - even though they looked the same there must have been difference in the piston - had to shorten the rod out of the power unit to get it to work...

Anyway -

Master cylinder changed. Brakes now work (pedal no longer sinks slowly to floor).

JUst been on first test run. Brakes work , but really only the rears (stamp on them and the rears will lock). Pedal is also unusually low (halfway down before take up).

Any ideas ? Pretty sure they are bled properly....

Morgan.

Be careful - it COULD be the push rod adjustment if the piston is not an exact like for like swop; BUT if you get it too long the brakes will lock on as the fluid heats up.

Also is the rebuild kit the "same way around" as the original? - i.e. is thecorrect reservoir feedin the front brakes - stating the obvious but could be a problem.

It could be that the front M/C ports are not bled out as much /as well as the backs.

Thanks Pete - that sort of backs up what I was thinking.
The push rod had to be shortened (screw adjust) for me to get the new masterter cylinder to pick up fluid properly. Its quite possible it now needs a little more length...

Ref straight swap. Its a new aftermarket master cylinder. Looks the same but for where the pipes fit the cylinder. My car exits engine side, Hemi version exits fender side. This is machined for both. (with one side blocked up).

Reservoirs , dimensions, even the cap shape all identical, so I am certain the big reservoir feeds the fronts. (I even tested both off the car to make sure the correct 'squirt' happened when piston pushed ! )

I will adjust rod a little and rebleed the fronts to be double sure.

it helps if you fill and bleed the resevior off the car or on the car and disconnected from the mian brake circuits.

but you need to make up a load of curly pipes to feed what comes out back in where the lid goes.

once well soaked in fluid (you the car and the master cylinder) put the lid on put it in more or less the right place and do up the proper brake pipes (each should have a bit of flex anyway for heat expansion compenstation)


also your brake peddle will have 2 positions for the pushrod (mines got 3???? bodger bill has been at it), one for manual and one for power brakes if you use the wrong position you get the same kind of issue you describe.

Dave

I've found none of these master cylinders actually need bench bleeding with pipes, just takes a little thought into whats happening.

If you try and bleed the master cylinder with it fitted to the car with the pipes on then on releasing the pedal it sucks air back in through the ports.

If you bench bleed it with pipes that are not under fluid level it still sucks air back in through the ports.

Here's how I do them, bolt it on the car but do not fit the pipes, get someone to slowly press the pedal down all the way, then put a finger over each port BEFORE the pedal is released, this way it can't suck air back in through the 2 ports so has to suck fluid in from the reservoir.

Just make sure a finger is over each port every time the pedal is released and they bleed easily, once fluid is there put the pipes on loosely, once more down on the pedal and tighten the fittings before the pedal is released.

Right at the start you may have to pump the pedal quite quick (with fingers over the ports) you will see bubbles appearing in the reservoir, this is just the 2 chambers getting a little bit of a prime.

All you're adding with your fingers is a tempory one way valve

bench bleed

http://www.moparuk.com/forums/viewtopic ... highlight=

I do what Neil W does

Also , don't forget the other Neils post either.

THanks as ever chaps - will have a go this weekend.

I am now thinking that perhaps the rod adjustment was a red-herring. At first I was getting no bleed at the rear at all, and decided it was because the piston was not picking up fluid (having not bench-primed or finger primed it).

A few experiments later, and a slightly sorter rod seemed to be the answer. Seemed to cure the problem, all 4 now bleed (but fronts of course dont work properly).

I think the course of action is thus -
- Adjust rod back to origonal length so I have a known start point.
- Bench/finger prime cylinder
- Rebleed and the re-assess.

Oh, forgot to mention, pumping with lid off seems to give a fountain of fluid. Probably rod length related...

Onward and upward - Its all a learning curve. No-one can accuse these cars of being non-interactive can they ?

Down in the dumps.

Everything now bleeds, rod at original length.
Brakes (when up on stands) appear firm. Front wheels lock when asked.

A few test runs back and forward seem OK. Venture out into the road. Few hard stops. Only seem to have rears. Any kind of 'firm' stop leads the rears to lock up - I cannot get the fronts to lock at all (yet they appear to work on the jacks).

Thoroughly fed up with it. Not that things are broken - I dont mind that, its just when things are broken and I dont know WHY. (which in turn means I cannot fix it)

As far as I can figure I am getting rear brakes before the fronts (hence why they work up on jacks, but not on the road) - my balence is all wrong. Given that
a) The ports are on correctly
b) the system is bled
c) The brakes work

I need to understand why I am getting rears on first which means that once they bite the pedal goes firm before the fronts have a chance to work. (or at least work with any force)

So - I have to admit defeat I think. Too many days and none of my variables seem to make the slightest difference. It may be that new mastercylinder is just knacked - I dont know. Is it possible that air is still in there somewhere ? The blleding is perfect - not the slightest 'fizz' left in (I must have run 2 litres through it ! )

Anyone recommend an expert in the herts area whose idea of a really good time is diagnosing the brake balance on bendix style master cylinders ? I mean a chargable diagnostic, not a freebie. I am at wits end on this.

Have you set the proportioning valve correctly? (if you have one)

Most master cylinders have a 70/30 front/rear , the rear one is normally for the fronts.

Adam68 wrote:Have you set the proportioning valve correctly? (if you have one)

Didnt know I had one... More research needed. Feel brighter this morning. After all - how difficult can it be ? Its only a master cylinder !

... ... ... ...

PS - Great avatar Adam !

you might not have one.

i don't and i can lock the rears quite happliy

my brakes work best when the rears are badly adjusted and my axle seal are leaking......!

no i don't suggest you try that

but but but

does the master cylinder have an electrical connection to it?? 1970 onwards on aussie cars

if it does then you may have a master cylinder equipped with s safety plunger.

unequal pressure in front or back circuit allows the piston to flip forward or backward to cut off the circuit that it suspects has a leak or is being bled vigarously.

i ran about with no rears at all for about 6 months until i discoverd the warning light taped to the inside of the dash to hide it.

reset the plunger (it is on a spring) by undoing the electric switch housing 8-10 mm then do it up again

Dave

Get a cheap Summit racing adustable proportioning valve. It fits in the line to the rear brakes and reduces the line pressure to the rear brakes.

Everyone should have one for three reasons.

1/ The factory non-adjustable unit was a "one size fits all" type of thing.

2/ The front/rear weight bias of your car as well as the width of your tyres is probably no-where near stock and these effect the brake balance.

3/ Badly biased front/rear braking can cause you to spin during an emergency stop.

Always bench bleed the master cylinder just as Neil has shown you in the photo above. In fact they are my brake lines he borrowed!

I struggled to get the air out of my master cylinder until I bench bled. You just get more air out when you force the piston in the cylinder to the very end of its travel by hand. You have to pump it quite a lot to get it all out as well.

BTW.

A smaller diameter bore in the master cylinder will take less effort to push but will travel further. A bigger bore has less travel but more effort.

Just a small fraction of an inch can make a big difference to the pedal travel and effort involved. This is more important with manual brakes of course.