*An update*
I probably should start a new thread but one of the possibilities is that I have too much sand in the filter so I've decided to continue this thread.
So I replaced a couple of bent laterals, the broken collector connector (that connects the lateral L shaped arm to the sand filter) and re-glued the air tube to the base of the lower lateral collector (and secured it a few inches up from the base with half a peg (as a spacer from the central pipe) and a stainless steel hose clamp. Just as well I did as I had to bend the air pipe to get the L shaped collector into the sand filter, I'm sure it would have snapped off again with out this extra support).
I tried to work out where 2/3 full would be on the filter and after a few measures inside and out I decided that 2" above the top inlet pipe (on the side of the filter) would be roughly 2/3rds. I added the two bags of gravel (3 to 5mm, label and manufacturer said 6mm but I couldn't find any 6mm, surely this can't be the issue? It only had sand before so any gravel is better than no gravel, no?), I cut a small corner in the bag and slowly poured it into the filter that I had half filled with water. I felt around the laterals after the first bag, it hadn't covered them so I pushed the gravel around a bit and the laterals I felt were still attached but I'll be honest, I didn't check them all sooo, maybe I missed one?
After adding back the old sand and closing everything up, I did a backwash and everything seemed fine. I then did a rinse and noticed that the glass viewer was full of sand. I remembered this happens with new sand so thought it might be because of the new gravel so repeated the process. Do a backwash and the sand disappears from the glass viewer, rinse and it is full of sand again. I did this 5 more times hoping it would clear up and even left the rinse going for 5 minutes a but the glass viewer still had sand swishing around it. To describe the rinse in more detail, the sand rushes in at the start of the rinse, swirls for a minute or so and then starts to thin out so what does this mean? If the sand in the glass is thinning out does that mean it's no longer pulling sand from the filter so does that rule out a broken lateral? I would have thought that if there was a broken lateral it would have continued to pull sand from the filter until it ran out of sand?
Now just when you were sitting there saying "it HAS to be a broken lateral", let me throw this into the mix. When I went to inspect the damage after the bracket broke and the flood gates opened, I also saw some sand in the glass viewer. I decided to open up the MPV and saw that the gasket (that comes fixed to the MPV manifold) was shot to bits. I ordered a new one and whilst I was waiting for the parts to fix the filter, replaced it. I struggled to find the gasket thing but eventually found a supplier in France and paid the high postage. Unfortunately the supplier forget to send the 2 washers so rather than wait any longer I reused the old ones. I sprayed some silicone lubricant on them (as seen on youtube) and the hole that the centre part fits into and it all seemed fine. The only thing is, when I put it all back together I noticed I didn't have as much leverage as I used to. After todays problems with the sand in the glass viewer, I took the MPV apart, sprayed some more silicone lubricant on the O rings and hole the rings go through and pushed them together. I noticed that the spring didn't push the two parts apart as much as it should have, seeming to stick on the upper O ring. This was working the day before, why is it sticking now?
Here's my problem though, I sit the new gasket head piece into the MPV (which is full of water, don't remember mine being like that, relevant?) with the open triangle at the top. I then place the washer, spring and washer onto the central column and then push the main MPV top plate down onto the new gasket central column. With one hand pushing down on the MPV top plate to depress the spring, I use the other to put the the 8 screws in the top plate and loosely fit the nuts below. I slowly tighten them with a screwdriver above and a small spanner holding the nuts below, crossing from one side to the other to distribute the pressure down on the spring. When it's fully tightened, I then grab the handle, drop it into position and find that I can easily slide the pin in from the right hand side. Now this shouldn't be this easy, when I first did this on my own, I had to hit the pin quite hard with a hammer and screwdriver to get it to slowly pop out the side. Because the pin goes in quite easily there isn't much in the way of leverage. I've got decent leverage on my own MPV but not on this identical one and the only difference is one has new washers and the other doesn't. Can washers make that much of a difference? They look absolutely fine so what is causing the lack of leverage?
Now I've been racking my brains trying to work this out but I can't get passed this bit. When I put the new gasket piece into the MPV it is sitting on the base over the holes that lead to all the pipes, it can't go any lower. When I place the washer/spring/washer on top and push the head piece of the MPV down onto the gasket piece, I am lowering the top MPV piece down onto the gasket piece and compressing the spring. Again, the gasket piece can't go any lower. So when I put the handle on, where has my lack of leverage gone to? Logic says that the gasket piece must have risen up into the MPV top piece and there is now a gap between the rubber gasket and the hard base where the holes to the pipes are. Now that I write this, I suppose as I'm wiggling the top piece down the rubber gasket piece could be rising up? Oh God!
More info, Just to see if the sand would return to the pool I turned the MPV to filter and started the engine. Straight away I could hear the sand bashing its way down the pipes to get to the pool so immediately turned off the motor. So rinse and filter are sucking sand from the filter but is it the gasket thing sticking open, an unlikely broken lateral or too much sand in the filter? (as I didn't know how much I had and may have over filled it). I'm beginning to think it could be the washers causing the gasket thing to stick open but quite how the sand gets through a perfectly working lateral to get the the MPV gasket thing, I've no idea.
Sorry for it being so long but I tend to think that more info is better than too little but I appreciate it can also put people off from reading it at all.