Disc/drum rears locking up?

[rski]

Problem Rears locking up.Front Baer serious street 2 piston calipers rear stock drums,Master 7/8 bore from 76 Monza . Master reservoir is reversed the front bowl is smaller then the back.Ran the back of the master to the front of stock junction (drum/drum) front of master to a wilwood adjustable proportion value then into the juction block.Bleeding the rears with the valve fully open we get very little flow and can hear sucking when bleeder is opened up.Fronts bleed fine.Even with the poor bleeding of the rears they lock up before the fronts,even with the proportion valve fully adjusted in. Had the same problem with a 1" master stock drum/drum junction rears would lock up first.



Any suggestion? Thanks

[Apogee]

Did you bench bleed the MC fully? You can confirm that it is fully bleed by plugging both outlet ports and pushing on the piston...if it strokes less than 1/4", you should be good. Is there a residual pressure valve in the front port of the MC for the rear brakes? Sounds like you've potentially got a combination of things going on.

Tobin
KORE3

[JRouche]

Did you re-build the rear cylinders? If not I would think about tearing them down and make sure they are not hanging up. You have some great brakes in the front. Might be worth a re-build for the rear cylinders including the entire shoe mechanics.

Im not all that knowledgeable about brakes. But what if the fronts just werent getting enough pressure. Lots of volume. But they need more pressure and less volume than the drums. So then the drums are getting all the volume they need and are actually working as they should, drum brakes really have some gripping power, they just tend to fade under high heat. Where the fronts arent getting the pressure they need and are not doing their part. Just an idea. JR

[rski]

Did you bench bleed the MC fully? You can confirm that it is fully bleed by plugging both outlet ports and pushing on the piston...if it strokes less than 1/4", you should be good. Is there a residual pressure valve in the front port of the MC for the rear brakes? Sounds like you've potentially got a combination of things going on.

Tobin
KORE3

Yes I did bench bleed the master.Not sure if there is a residual valve in the front port.I've never had a problem bleeding brakes 35 years worth.
I'm surprised the rear are working with how poorly they bleed.

Could this be a quick take up master like the S10's? Combination of other problems I'm ready to listen.And answer questions

Had no prolems with the rears when it was stock drum/drum 1" manual master so I know they are fine.

[rski]

Did you re-build the rear cylinders? If not I would think about tearing them down and make sure they are not hanging up. You have some great brakes in the front. Might be worth a re-build for the rear cylinders including the entire shoe mechanics.

Im not all that knowledgeable about brakes. But what if the fronts just werent getting enough pressure. Lots of volume. But they need more pressure and less volume than the drums. So then the drums are getting all the volume they need and are actually working as they should, drum brakes really have some gripping power, they just tend to fade under high heat. Where the fronts arent getting the pressure they need and are not doing their part. Just an idea. JR

yes rears are new see above.Fronts should be getting enough pressure with the 7/8 master.Yes good drums work great the 1st time after that fade is a big factor.

[Apogee]

The 1976 Monza MC you're using should be a straight 7/8" bore...too early to be a quick take-up style. The Monza used small, 11/16" bore rear wheel cylinders. You haven't said what your application is, but if it's the usual 67-69 F-body, those used 7/8" bore wheel cylinders out back, a full 60% more effective area than the Monza units.

Do you have a brake pressure gauge kit? I'd be curious to know what pressures you're getting in the rear with max and min proportioning.

Tobin
KORE3

[rski]

Tobin, Some very good info was over looking the wheel cylinder bore .Basically 1st gen f-boby 72 nova.Wounder how much if any I would be giving up switching to the monzas 11/16 wheel cylinder,would the master make up for it?I would think so.Maybe one more pc to the puzzle.I'll see if it's a direct swap.

I'll work on making up a pressure gauge,and get some info for your data base.

Thanks Rob

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