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PV, 120 (Amazon), 1800 General Forum for the Volvo PV, 120 and 1800 cars

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Soft compound brake pads Volvo 131?

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Old Jul 30th, 2020, 23:19   #11
fishyboy
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Thanks all.
Derek,
I thought that the cylinder size was dependent upon whether a servo was fitted rather than a PRV. That's certainly what the Brookhouse site suggests (https://www.classicvolvoparts.co.uk/...d_Twin_Circuit). I think my non-servo car should have the 1" cylinder. If someone put on a 7/8 would that cause higher pressure at the rears and explain my rear brakes locking first?

I guess I'm going to have to bite the bullet and pull off the drums and check the cylinder size. Last time I pulled the drums on my 1800 I messed up pretty bad (https://www.volvoforums.org.uk/showthread.php?t=304683). I'll pay more attention this time.

What is the "bore" that I measure to get the 7/8" or 1" size? Presumably the cylinder has to be removed before I can measure?

I've been also been looking into getting the front disks skimmed. I can only find one outfit nearby but they only offer skimming with the disks on the car service. Is this they way to go?

Phil
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Old Jul 31st, 2020, 11:52   #12
Volvo_133
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Hi Phil,
Have the discs sorted first then check if fault is rectified. To check cylinder bore size you need to measure the outside diameter of the piston. Check both sides of car incase they are odd.
To check piston diameter remove 1 shoe, have someone gently press the brake pedal to expose 5mm of piston and measure it.A tape measure should be accurate enough for the 3 mm (1/8 inch) difference that you are checking for. While your in there check the shoes for worn linings, cylinders for leaks also lubricate the brake adjusters with copper slip or graphite grease. Check the shoes are fitted in their correct format.
NS shoes ( front shoe should look to have lining missing at the top, rear shoe should have lining missing at bottom)
OS Front shoe lining short at top, back shoe lining missing at bottom.
My discs were removed and lightly surface ground. If someone can machine them in position
they must be machined to ensure the thickness is constant and front face is parallel with the rear face.If they are willing to do this I assume they are competent.
Chris
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Old Jul 31st, 2020, 12:02   #13
Derek UK
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I see what you mean with the Brookhouse listing. IMO they should say with or without PRV, not servo. A servo just gives you a lighter pedal that enables you to modulate the braking better it does not change the way the car brakes as far as the amount of pressure that is applied to the discs and rear drums. Yes, a servo makes it easier to lock up the rear due to the quick weight transfer to the front which takes the weight off the rear, if you hit the brakes a bit quick. This where the PRV helps.
Comments welcome.

7/8" and 1" is the internal bore but if you had both in front of you the 1" one would be noticeably bigger. When you just have one. a calliper measurement of the outside should help you make a decision.

Last edited by Derek UK; Jul 31st, 2020 at 12:06.
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Old Aug 1st, 2020, 21:41   #14
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Hi Derek
I've checked several other Volvo parts outfits on the Continent and they all mention that wheel cylinder bore size depends on whether a servo is fitted (1" no servo, 7/8" with a servo). I looked through the "Green Service Books" for my 68 1800S and for my 131 and I can't find any mention either.

The parts book list a brake valve (671894-4) which covers chassis numbers 193800 to 225049. Mine is 199984 so that fits. But I cant seem to find any thing that states difference bore size if a PRV is fitted. I'm guessing that all servo cars had a PRV and therefore should have the 7/8" cylinder. What I really want is something in a manual that says non-servo cars but with a PRV should also have the 7/8" bore cylinders.

Might try to call Simon at Brookhouse and see what he says.

Phil
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Old Aug 5th, 2020, 23:16   #15
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Hi all
Spoke with Simon at Brookhouse yesterday and he strongly believes that the 7/8" bore wheel cylinder is for the cars with servos and 1" for those without.

I think the real relationship may be that all servo cars should also have a Brake Restrictor Valve (BRV) as well.

My thinking is that a non-servo car, but with a BRV should also have a 7/8" bore wheel cylinder.

I'll let you all know what size my car has when I get the courage to remove the drums.

I really need to get to the bottom of this one as its uncomfortable driving a car when you know that the rears will lock-up before the fronts!

Phil

PS. I might ask the same question on "Swedespeed Forum" (https://forums.swedespeed.com) and see what answers, if any, I get.

Phil
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Old Aug 6th, 2020, 08:49   #16
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As I understand it, the difference between a 7/8" and 1" rear cylinder is as follows. A given distance of brake pedal travel will displace a given quantity of fluid, which will move the piston in a 7/8" cylinder slightly further than a 1"(due to volume behind piston). This in itself increases the braking at the rear relative to the front. So in effect changing rear cylinder diameter alters the front/rear braking balance, regardless of whether a servo or PRV is fitted. Just to confuse things, a larger diameter cylinder will be able to excert a greater amount of mechanical force for a given amount of pedal pressure. Thoughts?
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Old Aug 6th, 2020, 09:00   #17
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Pressure is Force/area, hence the bigger one can apply more pressure with less force applied and assume that is to compensate for no servo.
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Old Aug 6th, 2020, 09:43   #18
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Hi Underdrive and Burdekin

That's really interesting.

Relating that to my rear brakes locking up before the front disks does that suggest that my car has got 1" bore wheel cylinders which are applying too much force and locking up early. Therefore if I fitted 7/8" bore wheel cylinders applying less force the rears would have a lesser tendency to lock up early?

Of course I am assuming that the BRV is functioning correctly.

Phil
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Old Aug 6th, 2020, 10:49   #19
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But which would cause premature lock up, greater force or greater travel? I was thinking that the rear brake shoes might be contacting the drum fractionally sooner than the front pads with the 7/8" due to longer travel. I suppose only a 4 wheel brake tester would be able to check accurately, but surely this must be well researched and established braking theory documented somewhere.
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Old Aug 6th, 2020, 14:58   #20
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Quote:
Originally Posted by Underdrive View Post
But which would cause premature lock up, greater force or greater travel? I was thinking that the rear brake shoes might be contacting the drum fractionally sooner than the front pads with the 7/8" due to longer travel. I suppose only a 4 wheel brake tester would be able to check accurately, but surely this must be well researched and established braking theory documented somewhere.
The bigger piston will displace more volume and exert more pressure. I can't think why both wouldn't have a PRV unless there is some physical difference in the plumbing. Will go have a gander at the green book.
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