Issue with front brakes, master cylinder issue?

[Rustic76]

Hi guys. I'm trying to diagnose a problem I'm having with my front brakes on my Aussie spec '84 HJ47.

The pedal seemed a little soft And I also noticed when the road was wet, my back drums were locking up under moderate braking. I jacked up the front end and I am not getting any braking to the front discs. When the brakes are applied, the calipers grab for a second then release and the tire spins freely with the pedal to the floor. Even pumping the brakes there is no change..

- bled entire system with no change. Noticed the fronts, while bleeding didn't exert much hydraulic pressure.

-checked lines for a damage and all look good.

Is it safe to assume my master is bad?
It looks to be a pretty new unit.

Some advice would be appreciated.

Thanks, Justin

 

[78fj40mg]

Justin, it could be the calipers and/or the master cylinder. My philosophy is if there's a brake problem replace parts, life is worth more than $300.

 

[Rustic76]

I agree. The reason I suspect the master is because it simply doesn't seem like the hydraulic force is present. It seems to retract after the initial push of hydraulic fluid, even with the pedal still depressed. Where the heck is that pressure going?

 

[Racer65]

It is curious that the MC looks so new. Does it say Aisin on it?

 

[Rustic76]

It definitely looks new, It's not an Aisin though.

 

[whitey45]

Calipers are likely frozen, pull the pads and see if you can depress the pistons, easy fix as they are cheap and available anywhere!

 

[whitey45]

"]Calipers are likely frozen, pull the pads and see if you can depress the pistons, easy fix as they are cheap and available anywhere!
Or if your fed up , I will take it off your hands!!

 

[Rustic76]

@whitey45 Brian, Is my thinking wrong that there should be a good amount of hydraulic pressure to the caliper? Ill pull the calipers and take a look.

 

[whitey45]

I have had calipers seize and no amount of hydraulic presure could free them! Just pull the pads and using a small pry bar leverage the piston against the rotor, piston should easily retract.

 

[Rustic76]

@whitey45, I took off the left front caliper, 2 of the 4 pistons retracted easily, two needed a little more persuasion with a clamp. I re installed after a cleaning and still the same issue. Any ideas?

 

[whitey45]

Rare but could be the front brake soft lines, they can swell and contract, otherwise it has to be the master?

 

[nogo]

Has this been a problem since it arrived?......I'm thinking maybe someone has fitted the wrong master cylinder .

 

[Rustic76]

Has this been a problem since it arrived?......I'm thinking maybe someone has fitted the wrong master cylinder .

Yeah. Based on the master looking really new, I wouldn't be surprised if that's what happened. I may pull the lines and look at what valves it has in the front and rear outputs.

 

[DSRTRDR]

soft brakes point to leak or master cylinder problem, hard brakes point to booster

 

[nogo]

The drum/drum master cylinder should have a bore of 25.4mm (1") .... The disc/drum master should have a bore of 23.81mm (15/16")
The drum/drum master would give a vehicle with front discs a spongy pedal

The bore size should be cast on the master cylinder.

 

[Rustic76]

The only marking this master has on the body is B-31 and front/rear, no bore size like they typically have.

I also pulled the valves and this is what they look like, both identical. If I remember correctly, I replaced a 40 series master cylinder with a centric/auto store variety on a '76 and had to swap both of these valves with the old Aisin ones because it wouldn't work right. Maybe I'm on to something?

 

[nogo]

Was there a residual check valve under the spring on the front line?

 

[Rustic76]

Was there a residual check valve under the spring on the front line?

Here is what it looks like... only think I'm there looks like a small brass crush washer.

 

[nogo]

Pics of both springs you removed please.

 

[Rustic76]

@nogo These two valves from the other pic were threaded in, front and rear, no springs or anything else. Both identical in size, no rubber valves on either.