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Thread: Low rear brake pressure

  1. #1
    Member daveyboydave's Avatar
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    Low rear brake pressure

    Background:

    Just refitted refurbed standard rear brake calipers. Changed the seals and bench bled the BMC. ABS removed and braided lines run through the cabin. Tilton proportioning valve in line to rear brakes. All brand new, system bled.

    The brakes don't feel as sharp as I'd like but I'd put it down to needed a few more bleeds. But on getting MOT'd today the tested highlighted that the rears aren't providing anything like enough pressure. Fronts are fine. MOTer tried the proportioning valve on max, min and middle pressure but it made no difference; both rears were matched.

    So, any thoughts on the cause? My thought process so far:

    • Proportioning valve clearly not doing anything but this may be because it's not receiving the minimum pressure before it starts to reduce (noting these only have a restricting effect above a certain inlet pressure point, and the adjustment alters where that point is). Also, a bit of googling suggests they fail open. But still a possibility that I was sent a duffer.
    • Caliper issue unlikely since both rears refurbed and affected equally.
    • Could be a problem with the rear piston in the BMC?
    • Blockage in the lines between BMC and the rear t-piece?
    • Don't think it's a bleeding issue as that would affect the entire hydraulic system and therefore also the fronts.


    Thinking about getting a pressure gauge I can plumb in at various points to try and narrow down the issue, but before I do so has anyone seen anything like this before, or have any thoughts?
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    Member daveyboydave's Avatar
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    I should say, the BMC is a BM57 (always worked fine before the refurb) and I didn't touch the proportining bit if it has one (I just pulled the pistons and springs and changed the seals).
    Last edited by daveyboydave; 21-08-2020 at 22:03.
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    Member daveyboydave's Avatar
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    Well for those unable to cope with the suspense (blimey SXOC is quiet these days!), I removed and disassembled the BMC and can't see anything wrong, no damaged or reversed seals or anything. I haven't broken into the internal proportioning valve because I dont want to disturb it and complicate matters when I've never heard of them failing.

    20200820_210345 by DaveyBoyDave, on Flickr

    Next step is to rebuild, bench bleed and refit the BMC, rebleed the system and test to see if I can feel any more action from the rear brakes suggesting it's magically fixed itself. If not, I'm going to get a coupler and bypass the Tilton bias valve to rule that out.

    Grateful for any alternative suggestions anyone might have.
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    its certainly a strange one.
    you could buy a cheap universal pressure gauge and plum that in but im not sure wheat the pressure should actually be.

    I had an old transit with the same issues and I never got to the bottom of it so totally feel your pain.

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    Member daveyboydave's Avatar
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    Quote Originally Posted by green_rs13 View Post
    its certainly a strange one.
    you could buy a cheap universal pressure gauge and plum that in but im not sure wheat the pressure should actually be.

    I had an old transit with the same issues and I never got to the bottom of it so totally feel your pain.
    My first thought was to do this, to be able to tell what the pressure was before and after. But I've struggled to find a cheap gauge that can cope with around 2000psi. Best I've come up with is the following on ebay:

    https://www.ebay.co.uk/itm/3Brake-Pr...wAAOSwvD5Z82vs

    But 92 is more than i'd prefer to spend given I can't see me ever using it again. I think I'll have to check it by feel, or alternatively I suspect if I bung my MOTer 10 he might check it on the rollers again for me.
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    I would say get rid of the proportioning valve to start as you are going to do, remember its a single line to the rears that are being split so less than optimal to start with, any orifice that's dropping the pressure even more isn't going to help.

    Are you running bigger than normal brakes on the front to go with the bm57? If not thats 99% of your problem right there as it's designed to work with bigger callipers so probably has more pedal travel now the abs pump isn't pumping the fluid through. Plus your on stock rears which are smaller than designed for that m/c....

    I have been running stock m/c with no abs for years now with no problems but that's with stock front and rears, did upgrade to z32 alloy calipers on the front but they are still same size pistons etc.

    Good luck and let us know how ya go,

    Cheers, Dan
    Last edited by drc; 24-08-2020 at 22:49.

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    Member daveyboydave's Avatar
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    Quote Originally Posted by drc View Post
    I would say get rid of the proportioning valve to start as you are going to do, remember its a single line to the rears that are being split so less than optimal to start with, any orifice that's dropping the pressure even more isn't going to help.

    Are you running bigger than normal brakes on the front to go with the bm57? If not thats 99% of your problem right there as it's designed to work with bigger callipers so probably has more pedal travel now the abs pump isn't pumping the fluid through. Plus your on stock rears which are smaller than designed for that m/c....

    I have been running stock m/c with no abs for years now with no problems but that's with stock front and rears, did upgrade to z32 alloy calipers on the front but they are still same size pistons etc.

    Good luck and let us know how ya go,

    Cheers, Dan
    Thanks for the thoughts Dan.

    The setup is R33 GTR fronts and standard rear calipers, with the BM57. This is what came on the car when I bought it about 8 years ago (I didn't even realise the BMC was non-standard until I refurbed it last month).

    I can see how this may not be absolutely optimal, and one of the reasons I took the opportunity to fit the proportioning valve is to be able to tweak the balance to refine it if need be. (It might be that at some point I shim the internal BMC proportioning valve to allow the external valve to be able to have more control and set a higher pressure to the rears to compensate, but that's future Dave's problem, I just want to get it working as it was for now!)


    That said, it's been absolutely fine all this time, on multiple track days. I've taken it to the same MOTer every time, and he knows what he's talking about (FCM Motorsport), and this is the only time it's had a problem with the rear being under-braked on the rollers. Which is why I'm so certain it's something I've done or fitted during the ABS delete and general refurbishment, either through defective parts or idiocy on my part!

    With regards the splitting of lines, with the ABS BMC there is only a single outlet for front as well as the single for rear, so both pairs are fed via a T-piece and therefore I don't think it's an issue. The amount of fluid flow (noting pressure is the key point not mass flow rate, but whatever) is the same as before I've done the recent work, I've just removed the ABS pump and about 50% of the line length from all the plumbing in between.

    Last night i refitted the BMC and re-bled the system. Took it for a test drive and the pedal feel was ok, could be firmer. Braking was reasonable but I could easily lock up the front left wheel and no other. Feeling the disc temperatures afterwards, both fronts felt reasonably equally hot and the rears had some temperature, so all 4 brakes are doing something at least....

    I'm starting to think the issue might just be poor bleeding. Until last night, but since the ABS delete, I was getting some creaking from the front left caliper, which I put down to maybe a sticky piston. After last night's re-bleed the creaking was gone and it was the corner that locked up. Makes me think there was some air still in one side of the caliper, and the asymetric clamping force was causing the creak.

    Anyway, chatting with Jon at FCM, he says when flushing and filling a new system he might put 2 litres of brake fluid through. I've probably used about a litre so it might just be there's still air trapped at certain points causing certain corners to be weak. (I'm also dealing with the age-old problem of a wife who gets bored pushing the pedal for the 50th time.....)

    I've just ordered some Goodridge speed bleeders and some more brake fluid, so I can bleed single-handedly to my heart's content without incurring my wife's wrath. Plan is to fit the those and absolutely bleed the shit out of everything, like 10 times, until I can completely rule out air as a root cause. Perhaps not a surpise that with an entirely new braking system, it's going to take some perseverance to get every last bit of air out. The BMC is almost certainly good, the calipers are good, the lines are just lines, and the Tilton proportioning valve should only ever fail open, so air feels like the most likely remaining candidate.
    Last edited by daveyboydave; 25-08-2020 at 08:25.
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    Quote Originally Posted by daveyboydave View Post
    Thanks for the thoughts Dan.

    The setup is R33 GTR fronts and standard rear calipers, with the BM57. This is what came on the car when I bought it about 8 years ago (I didn't even realise the BMC was non-standard until I refurbed it last month).

    I can see how this may not be absolutely optimal, and one of the reasons I took the opportunity to fit the proportioning valve is to be able to tweak the balance to refine it if need be. (It might be that at some point I shim the internal BMC proportioning valve to allow the external valve to be able to have more control and set a higher pressure to the rears to compensate, but that's future Dave's problem, I just want to get it working as it was for now!)


    That said, it's been absolutely fine all this time, on multiple track days. I've taken it to the same MOTer every time, and he knows what he's talking about (FCM Motorsport), and this is the only time it's had a problem with the rear being under-braked on the rollers. Which is why I'm so certain it's something I've done or fitted during the ABS delete and general refurbishment, either through defective parts or idiocy on my part!

    With regards the splitting of lines, with the ABS BMC there is only a single outlet for front as well as the single for rear, so both pairs are fed via a T-piece and therefore I don't think it's an issue. The amount of fluid flow (noting pressure is the key point not mass flow rate, but whatever) is the same as before I've done the recent work, I've just removed the ABS pump and about 50% of the line length from all the plumbing in between.

    Last night i refitted the BMC and re-bled the system. Took it for a test drive and the pedal feel was ok, could be firmer. Braking was reasonable but I could easily lock up the front left wheel and no other. Feeling the disc temperatures afterwards, both fronts felt reasonably equally hot and the rears had some temperature, so all 4 brakes are doing something at least....

    I'm starting to think the issue might just be poor bleeding. Until last night, but since the ABS delete, I was getting some creaking from the front left caliper, which I put down to maybe a sticky piston. After last night's re-bleed the creaking was gone and it was the corner that locked up. Makes me think there was some air still in one side of the caliper, and the asymetric clamping force was causing the creak.

    Anyway, chatting with Jon at FCM, he says when flushing and filling a new system he might put 2 litres of brake fluid through. I've probably used about a litre so it might just be there's still air trapped at certain points causing certain corners to be weak. (I'm also dealing with the age-old problem of a wife who gets bored pushing the pedal for the 50th time.....)

    I've just ordered some Goodridge speed bleeders and some more brake fluid, so I can bleed single-handedly to my heart's content without incurring my wife's wrath. Plan is to fit the those and absolutely bleed the shit out of everything, like 10 times, until I can completely rule out air as a root cause. Perhaps not a surpise that with an entirely new braking system, it's going to take some perseverance to get every last bit of air out. The BMC is almost certainly good, the calipers are good, the lines are just lines, and the Tilton proportioning valve should only ever fail open, so air feels like the most likely remaining candidate.

    Ah I see, good that the MOT guy mentioned it then and didn't just brush over it, especially if he has done the car before as you say. Last thing you want is air in the system when slamming on the brakes hard on track! Keep us posted then, sounds like you have it down to just needing more bleeding from your reply so hopefully that's all good, I must admit with stock brakes, with or without the welded diff I have never locked a rear wheel under braking on drift days or on the road, unless intentional with the handbrake.....

    Also on the lines what I was trying to say is that on my stock system I just bridged the pipes where the pump was so the rest of the lines should still be stock-ish length, and ye all the BMC's of that age are single front and rear ports split further down, not a problem but compared to a more modern twin line system the master cylinder has to move more fluid or should I say you need to move more piston (foot) travel to get the same as stock. IMO obviously but I am in the trade so do have some experience replacing master cylinder, brake pipes, bleeding etc.

    Good luck

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    To follow this up, I installed the speed bleeders and bled everything to death. Brakes feel more positive now and seem to lock up the fronts equally. Got a track day in a few weeks so that will be a more thorough test.

    So I think the brakes are working as well as they did before I played around with the lines and BMC, although that does leave me with under-braked rears, albeit now hopefully not so bad as to gain an advisory at MOT time. It seems that underbraked rears is a fairly common issue (even with, say, Z32 setups) according to lots of searching.

    My options, as I understand it, are to either to get biggers calipers / disks at the rear, or get the standard rear calipers working harder. I suspect the fronts overheat and fade and long way before rears would and therefore it seems there is thermal margin to spare, so my preference is the latter - not least since I've just had my rear calipers expensively refurbished! This also avoids adding extra weight and rotating mass as another bonus.

    I've found that others have locked out the internal proportioning valve in the BMC, relying instead on an external valve to set a higher pressure knee point and therefore supplying more pressure and breaking force to the rears. Since I have the tilton proportioning valve fitted this seems like the logical next step. Other things to consider are more agressive pads on the rear (I currently have Ferrodo DS2500s).

    Alternatively, I'm sure it must be possible to get spacers to use standard calipers with bigger discs, thus providing greater breaking torque (and more thermal mass), at the expensive of weight, but I can't seem to find a kit online.

    Will see how it goes at Blyton Park first and then think about next steps.
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