Classic cars forum & vehicle restoration.

[AVRO]

Hi all,

This is my first post so go easy on me.

I have recently been asked to get a Commer B series Superpoise APU to a drivable state after sitting since the early 80's.

Sounds easy enough.........

Strangely enough, it's only done 8000 miles from new (i imagine getting abused around the airfield) but with a quick clean of the carb, fuel pump, points, few engine grounds and some fresh fuel it started more or less the first time with little problem.

So next on the list was the brakes, it has drums all around with every wheel cylinder seized solid, brake hoses had all internally collapsed and the Master cylinder's seals were well on the way to return to the earth as they say (the seals had turned into black goo).

Ok, so moving on i've now replaced every seal and flex hose on it (brake shoes and Drums were in surprisingly good condition). My problem started when i came to bleed the brakes, someone had adjusted the pedal so the master cylinder never equalized the system (which took me a while to spot), on top of that the recuperating holes are so small it took two days to get the system bled to a point i was happy with and even then i found i had 3/4 of the pedal travel doing nothing before going solid near the floor locking all four wheels (that's with each corner set so you could just feel the shoe touching the inside of the drum without grabbing).

At this point, i did a fair bit of reading up (i have little experience with rear drums let alone drums all around). I came to the conclusion i needed to adjust all the brakes over tight (so all four wheels were locked up on the adjusters) bleed the system and then adjust it back off...... This didn't help.

Next i clamped off the front flex hoses so just the back brakes were working, this reduced the pedal-free travel to 1/2, now thinking i was on to something i clamped off the rears so only the front worked and this gave me 1/4 free travel in the pedal (which I suspect is how it should be).

So i took apart the rear brakes, stripped the rear wheel cylinders, and reassembled without really finding anything wrong, i made sure the cylinders were in contact with the shoes so there was no free movement between the two, and it was all assembled to diagrams in the workshop manual just in case someone had been in before me...... once reassembled it was exactly the same (3/4 free pedal travel with the last 1/4 being solid and it didn't pump up). The only thing i could fault on it was the springs inside the wheel cylinders that hold the seals square looked a bit weak.

The odd thing is if i push the brake pedal down by hand i can feel there is some resistance there although the brakes don't do anything until i hit the last 1/4 (it's like the shoes are way off the drums and you have to push them out). To add, i thought an adjuster may have been stuck, so i have taken them all apart, cleaned, greased, and reassembled (all work fine).

Once i get my head around that it would seem the clutch also bites right at the top of the pedal, but it bites so hard it makes the rear axle tramp when trying to move away slowly in first and reverse (next to impossible to roll the clutch, like an on/off switch) on visual inspection it looks to have plenty of meat on it (which is odd).

But one thing at a time...... can anyone suggest why with a well bled and adjusted drum setup that doesn't pump up I'm getting so much free pedal movement??

I was starting to think someone may have fitted the wrong size master cylinder at some point, but fortunately, the manual has the part number which is clearly stamped into the master cylinder...... as for the fluid, it says in the manual it should have Castrol crimson fluid but I've had to use Castrol Dot 4 which i believe should work fine. I'm now getting people saying 'ahh that's how it must be' but i don't think it is somehow.

Yes most entertaining........ and with it being in a museum I only get Tuesdays to work on it.

[Ray White]

Hello AVRO.

It is always possible that someone has been at the vehicle before you and left it worse than it should be.

My first (and probably wrong) thought was that the rear wheel cylinders have too small a diameter. Have you checked that they are correct for the vehicle?. They may have been replaced with an incorrect size.

[petelang]

Not sure how you are bleeding the system but I had a three day pain trying to bleed a Daimler after replacing a master cylinder. Initially I tried an Ezibleed but the cap didn't fit the reservoir to resorted to sucking fluid through as was doing this single handed. I then tried leaving bleeds open, tube immersed in fluid, and pumped pints of fluid through but only got a partial hard pedal. Giving up and enlisting an assistant, resorted to good old fashioned "up, down, close pedal up, down, etc, etc.
After another few pints it finally came right, hard pedal just after the free play. My assistants knee was hurting by then but a result was achieved. Been perfect ever since.
Sometimes, the old way is the only way.
_________________
Daimler Fifteen 1934
Armstrong Siddeley 15 Long 1933
Daimler V8 250 1969

[Ray White]

Peter... he claims to have bled the brakes...

If the Master cylinder has a worn bore, new seals may not be sufficient. The low milage (not unusual for military vehicles) would indicate, however, an issue with corrosion due to lack of use. I would replace the master cylinder or if that is not an option, have bored and lined in stainless steel.

[AVRO]

Thanks for the replies guys.

Ray, i haven't quite got my head around how the cylinders are sized to be quite honest with you, i noticed the rears are 1 1/4" where the fronts are 1". My initial thought was the rears were too big?? but it turns out they are correct (according to the manual)..... i can only assume they have done it like that to make the fronts quicker to respond than the rears as there is no bias valve on the back axle.




Pete, That's interesting..... so i have tried everything i could think of, the best method was actually putting a pressure bleeder on the reservoir and then pumping the pedal a few times and cracking the bleeder with the pedal up (i know counterintuitive but the recuperating holes in the master cylinder had to be open). I think I've put about three liters through it now (starting to be able to taste brake fluid through my skin thanks to gloves that last about five seconds ) but i will try doing it the old school method now it has some sort of pedal. What's odd is I've not seen any air bubbles for about half a liter of fluid, if i didn't know better i'd say it was fairly well bled but it's very possible it's got some sort of airlock.

Thanks,
Chris

[AVRO]

[Ray White]

I may be barking up the wrong tree but I would think you could go from a 1/4" down to 3/16" ...but the other way round??? I don't know what would happen if you increased the diameter of the pipe from the m/c to the dist. block?

I honestly don't know the answer.

Perhaps 'Classic Brakes' might have a suggestion.?

(Well done for having the M/C tested by the way. )

[AVRO]

I did ask Classic brakes for any suggestions, they did give me a few things to look at but i think even they were a little puzzled.

Yes the 3/16 pipe does bug me a bit and might be getting changed in the near future, but i don't think that would cause my problem??

[bjacko]

Sounds like the brake pedal itself needs adjusting to remove most of that "free" play or the linkage to the master cylinder.
_________________
1938 Morris 8 Ser II Coupe Utility (Pickup)
1985 Rover SD1 VDP

[Keith D]

AVRO,

I think Bjacko has a good point.

I had mega fun and games when I restored the complete braking system on my 1926 Chrysler almost 20 years ago. That car was fitted with hydraulic brakes from new, but they were rather crude compared with modern systems. (External contracting!) Like you, I had a problem with far too much pedal travel before the brakes engaged. I replaced the linkage that connects the brake pedal to the master cylinder piston with a longer one, making the cylinder piston move earlier. The brakes then operated with far less free travel of the pedal. (you must ensure that the master cylinder piston retreats enough to fully release the brakes.) Result, almost 20 years of completely trouble free braking.

Obviously I don't know if this would fix your problem, but it is worth a look.

Keith
_________________
1926 Chrysler 60 tourer
1932 Austin Seven RN long wheelbase box sedan
1950 Austin A40 tourer
1999 BMW Z3
Its weird being the same age as old people.
You are either part of the problem or part of the solution

[Ray White]

[AVRO]

Thanks for the replies.

I had eliminated the pedal assembly because i rigged up a short pipe with a bleeder in the end and then fitted that to the master cylinder effectivle making the brake system 3" long with no cylinders, it was originally to make sure the master cylinder was comprehensively bled of air but the side effect was the pedal went solid right at the very top of its travel.


I might be confusing things with my use of 'free play', i have set in the amount of free movement to the pedal to make sure the piston in the master cylinder is returning fully back to expose the recirculation holes. If i remove the pedal assembly so i'm pushing the rod into the Master cylinder by hand there is a fair amount of resistance to about halfway down the travel where it becomes too hard to push any further by hand. The resistance between 0 and 50% doesn't get any more or less, it feels just although you have one of the drums off.... but they are all in place and the shoes are all adjusted up a little tight if anything and the wheel cylinders are touching the contact points of the shoes.

I'm sorry if i'm making a meal of explaining this to you, i fully understand it's difficult to diagnose a problem without having eyes on it. Hell, i have hands-on and it's still confuses me

[Ray White]

One problem we have is that brakes can appear to be counter intuitive. I have read elsewhere that if for example the diameter of a cylinder is reduced, the pressure is increased (and vice versa). If therefore you increase the size of your master it may make your problem worse.

What confuses me is that many drum braked vehicles have bigger wheel cylinders at the front than the back but your Lorry has bigger rear wheel cylinders at the back - presumably to handle extra weight - but again if smaller cylinders give higher pressure then to me it is counter intuitive.

[AVRO]

This may sound like a bit of a reach, but as i said earlier the springs inside the wheel cylinders are not great (they have been compressed for 40 ish years) and they don't have a huge amount of resistance... so my theory is the springs aren't strong enough to keep the seal pushed against the back of the wheel cylinder piston??

The seals are not fixed to the piston in either the front or rear cylinders, the seal sort of floats and is held against the back of the piston by said spring.

I hope this link works, it's a similar cylinder.

https://www.2carpros.com/images/question_images/209877/original.jpg

[AVRO]