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What causes brake pedal to go to the floor in a 1975 HJ Holden

I have recently bought a...

I have recently bought a 1975 HJ Holden one tonner with power assisted front disc brakes and drums on rear.
The brakes work very well most of the time, however on a number of occasions the brake pedal has gone to the floor and have had to be "pumped up" again. It happened once after negotiating a roundabout and then four times after reversing down the driveway onto the street and once proceeding down the street, the pedal went to the floor. Once "pumped" back up, the brakes are excellent.
The master cylinder looks very new, I've completely bled the whole system, the disc pads and rear linings look OK, the rear drums are adjusted up correctly, the handbrake works as it should. The brake pedal does not go down gradually when pressure is applied for a long period and is not spongy. I haven't been able to verify that both pistons are working on the rear wheel cylinders but my next step was to replace them anyway.
Anyone else come across this and what are your ideas.

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Answered in 2 hours by:

7/8/2010

AU Auto Mechanic: TTVN, Performance tuner, Diagnostic tech replied 12 years ago

TTVN, Performance tuner, Diagnostic tech

Category: Australia Car

Satisfied Customers: 184

Experience: Motor Mechanic for 18 years, specialising in diagnostics, tuning, engine building, LPG ,Turbos etc

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Hi mate
there are really only 3 possible things that could cause your problem

1 is the master cylinder it is most likely the culprit however if it is the problem a quick way to tell is to unbolt the 2 nuts off that attach it to the booster and gently retract it off the booster and look in behind it to see if it is leaking fluid out the rear of it and is dribbling out either into the booster or down the front of the booster

2 load compensating valve above the diff can cause this problem as well and can be intermittent as you describe

3 it is possible that the rear wheel cylinders are sucking air back in which is not common but i have had it happen several times where they suck in air but wont leak fluid....... this is often found when bleeding that you bleed the system up and its all good then use the brakes a few times and then on the re bleed there is air straight up but no more after the initial bit

Plan of attack

first i would check the back of the master cyl as it costs nothing

second i would replace the rear wheel cylinders as they cost the least to replace and are pretty high on the problem list ...... you should get change from $50 for the pair

thirdly the load compensation valve should be rebuilt or replaced with known good unit

cheers Rod

TTVN, Performance tuner, Diagnostic tech

Category: Australia Car

Satisfied Customers: 184

Experience: Motor Mechanic for 18 years, specialising in diagnostics, tuning, engine building, LPG ,Turbos etc

Verified

TTVN and 87 other Australia Car Specialists are ready to help you

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Customer reply replied 12 years ago

Hi Rod

Just for your info so you can share with others.

I replaced master cylinder and rear wheel cylinders, checked operation of front disc calipers, adjusted and rebled whole system - (there is no load compensation valve) - no change.

Was at a meeting of car fanatics who commented about after converting HJ front brakes to their Torana, had same symptoms as mine and it was the brake lines to the calipers being a fraction too short holding back the caliper and so the pedal goes to the floor.

I checked this on mine and they were OK, but in the process found the tiniest scrape mark on the top of the calipers. Because my front end has been lowered (lowered springs) the top of the calipers are held back by the upper control arms lip when on full lock, therefore the pedal goes to the floor on the next application.

I simply hammered up the lip a little and ground a little of the top of the caliper to give 5mm clearance and problem solved

Thanks for your advice anyway - this problem nearly had me beat and it obviously had the previous owner beat, because I got the car for a song - he never mentioned the brakes - I found out the hard way!!!!!

Regards Greg

AU Auto Mechanic: TTVN, Performance tuner, Diagnostic tech replied 12 years ago

Great to see you got it sorted out mate

Thanks For the update , will log that curly one in the memory bank for sure

cheers Rod

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