Sticky rear brakes

[mylittletony]

Hi gang

I have an aftermarket rear disc conversion on my 57 Fairlane and after about 10 miles driving or so, they start squeaking and the wheels become hot to the touch, noticeably moreso than the front wheels.

Everything is still very shiny and new. Pad wear appears even. I can't ever recall using the handbrake, but even so, it returns to the stops and the cables are not tight. The sliders seem clean and free.

Is there anything else I should be checking?

The seller informed me the calipers are from an 85 Cadillac - I think it's a pretty universal USA spec upgrade...

It's pretty annoying and I'd love to get to the bottom of it

[HARDCORE]

Are your sliders suitably greased?

[zeberdee]

Did it have drums originally ??

I could be wrong , but i think some drum systems have a valve that always keeps a slight pressure . Something to do with keeping the shoes out near the drum ??
When doing a disc conversion , the valve needs removing as a caliper/disc doesn’t need constant pressure to work efficiently straight away .

As i say , I could be completely wrong , it’s just something I think I might of heard about .

👍

[Ritchie]

I’d identify the line going to the rear on the master cylinder, then when they are binding, crack the line at the master cylinder to see if there is any trapped pressure.
Could be an issue with the master itself or possibly the wrong length of pushrod preventing it returning fully and thus trapping pressure.

[zeberdee]

Found it .

Has it got a check valve inline ??

help.summitracing.com/app/answers/detail/a_id/5297/~/what-is-a-residual-valve%3F

👍

[mylittletony]

Jun 14, 2023 15:43:28 GMT zeberdee said:

Did it have drums originally ??

yes, but the whole brake system was replaced with a complete kit sold as one.

There is a chance the pushrod for the master could be a touch long, or they supplied a disc/drum master, but I wouldn't know how to check that...

To confuse matters, I can quite easily push the car (best part of 1700kg) when in neutral - so they're not exactly binding on, more like rubbing enough to generate heat and noise.

[brachunky]

Jun 14, 2023 15:37:24 GMT HARDCORE said:

Are your sliders suitably greased?

On this point, the sliders could be overgreased preventing them from operating properly. The grease can't be compressed so the sliders/housing is packed with too much, it may cause binding.

[Dez]

I fitted this kit, so can answer some of the questions. I’m helping Tony with this behind the scenes.

The kit was a typical American aftermarket ‘all in one’ 4 wheel disc conversion, which is a mixture of OEM parts from the GM stable, and various lazercut adapters to make them fit, as is the fashion over there.

It used a typical GM twin bowl master, they’re generally referred as Camaro ones but they came on everything. These don’t have integral RPVs, they’re in the bias valve block that is mounted directly below the master. The block is meant to be disc/disc. It needs some part number crunching to see if that is the case. I have a suspicion it could be a standard disc/drum one so the rpv to the rear line will be too heavy.
The master and booster are fitted and adjusted as per the instructions with the free play it specified.

Both ends us GM ‘metric’ calipers. These do not have conventional sliders as such. They have a nylon bushing in the caliper body that the slider pin (which just bolts into the lazercut bracket) goes through. As such they should not be greased as the dirt then sticks to the grease then wears the nylon out.

The front kit went on ok but did require some bracket grinding iirc. The rear had a few questionable bits, bolts not long enough to engage nylocks, wrong size copper washers, that sort of thing. Which I corrected as part of fitting. The major thing is that the rear pads overhang the discs and nothing much can be done about that, but it also shouldn’t cause any real issue as the pads would be totally gone before the overhangs touch each other as it’s a vented disc.
I think I also had to bleed the rear brakes with the calipers half off as the calipers are rotated a bit and the bleed nipple is in the wrong location.

It could be as simple as a bit of air in the system due to the poor caliper location, or is most likely an incorrectly specced valve block with the wrong rpv as previously mentioned.

[mylittletony]

Jun 15, 2023 14:08:00 GMT Dez said:

I fitted this kit, so can answer some of the questions. I’m helping Tony with this behind the scenes.

Thanks Dez and thanks for the help behind the scenes too!

I grabbed a pic of the proportioning block earlier, which shows some numbers.



Googling the 'PV4' marking, every reference suggest it's a disc/disc version. It seems disc/drum are PV2 from what I can see.

I won't have a chance to do anything until the weekend, but will get the rear wheels off and check the sliders either have a bush and are clean, or lightly greased if not. I'll also check the pads move freely.

As Dez mentioned I did notice that the pads don't sit very well radially on the disc, a result of the hotch-potch of parts used.
Because of this, they already have a lip on the outer edge, which may be rubbing on the outer diameter cylinder of the disc.

I'll try and chamfer the pads to prevent this, it's currently my prime suspect. It fits with the squeaking which isn't constant, but once or twice per revolution and would occur once some heat is in the disc, but not enough friction to prevent rolling.