Spongy Brakes

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Hi Folks
I have a VW Golf 1.6i 1998 AEE engine code. For some reason or another car has always had spongy breaks even after new discs and pads and new Brake shoes had been fitted! They have been bled on numerous occassions but the Good Pedal Feel factor and braking soon disappear in time. I have heard someone say replace the Master Cyclinder, but when the pedal is depressed there are no air bubbles in the Brake reservoir!! I would be grateful for any advice that can be given as to what the problem is! Many Thanks.
 
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Spongy Brakes you say?

You sure its not just the servo assistance working extremly well?

As you have bleed the brakes several times we can rule out a problem with air in the system.

Try this. Deplete the servo (engine off, pump pedal several times) press the pedal as hard as you can. Can you feel the pedal "creep" ie move downward as you apply a constant pressure? Start the engine, servo will charge and the pedal will sink. Keep same pressure on. Can you feel the pedal creep? If you get creep at any point whilst maintaining constant pressure then you may have a master cylinder fault, especialy if the pedal travels all the way to the floor.

The creep may be caused by worn seals in the master cylinder and the pressuriezed fluid returning past them
 
With the engine shut off and you depress the pedal several times, the pedal gets very hard. When the engine is turned on the pedal does depress a little but still feels spongy after a minute or so. Once when I had to braek suddenly the pedal went near enough right down to floor! Many Thanks
 
Was your brake bleeding done with a vacuum or pressure type of device or,
was the fluid pumped through the system by repeated depressions of the brake pedal ? A nine year old vehicle is likely to have some corrosion in the bottom part of the master cylinder bore, ie, where, in normal use, the main piston/valve assembly never travels. It is possible that the piston seal/s could have been damaged by passing through this unpolished part of the cylinder bore and are now allowing some microscopic leakage of fluid. Renewing the seals (If obtainable) or, a replacement master cylinder is the answer and, when bleeding the brakes in future, a block should be placed under the brake pedal so that the piston travel is limited. Also, have you changed the brake fluid or had it tested? These fluids (except the synthetics) are higroscopic and they attract water. If there is even a small amount of water in the system, the temperature, at the discs, will turn this water to vapour and, as this vapour is compressible, unlike the brake fluid, the brakes are likely to feel spongy.
 
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Spongy Brakes you say?


Try this. Deplete the servo (engine off, pump pedal several times) press the pedal as hard as you can. Can you feel the pedal "creep" ie move downward as you apply a constant pressure? Start the engine, servo will charge and the pedal will sink. Keep same pressure on. Can you feel the pedal creep? If you get creep at any point whilst maintaining constant pressure then you may have a master cylinder fault, especialy if the pedal travels all the way to the floor.

The creep may be caused by worn seals in the master cylinder and the pressuriezed fluid returning past them

Nitronick, you are more likely to get the pedal creeping down if you use light pressure on the pedal, using high pressure will force the seals onto the cylinder bore.

Watchmaker, the temperature needed to boil the fluid even when slightly contaminated with moisture is generally higher than most people ever attain.
 
Surprisingly, No. It is the water which boils, not the fluid, and the boiling point of water is easily reached, at the calipers, in some of those situations which are attained in "normal" motoring. This is why there are recommendations,by the motor manufacturers, that brake fluids are changed on a regular basis.
 
Make sure the load sensing valve on the rear axle is fully open when bleeding.
If it's got rear disc brakes (not sure if the 1600 did or not) take the calipers off the carriers, and bleed the brakes with the bleed nipple facing upwards. Silly design of putting the bleed nipple on the side of the caliper can get air locked in it however many times you try to bleed it with caliper attached.

If it has abs it may need bleeding with the car plugged into VAG-com.
 
I think unlikely after all it's only 9yrs old, but for what it's worth, I have had the flexible hoses "weaken" to produce excessive travel - but not intermittently.
MikeD
 
Watchmaker, I know all about brake fluid, I worked for 25 years as a senior road test technician for a large brake lining manufacturer in West Yorkshire. I once had the opportunity to go testing in Austria on the Gross Glockner pass, doing work for Ford, after a descent of the pass we had to park up and wait until the brake fluid reached it's maximum and if it exceeded 160deg c then it was classed as a fail. Ford had got there dealers to take samples of customers brake fluid and the average boiling point was 160deg c, so I think your theory is invalid. The only time you should ever get any fluid boiling is if there is a problem with the braking system or your a chav and drive like an tw*t.
 
"so I think your theory is invalid"
Not MY theory but, exhaustive tests, carried out by certain major oil companies, the Police. and motor manufacturers, resulting in much publicized data revealing the previously unknown danger of water absorption by brake fluids. It surprises me that your claim to know ALL (Really ?) about brake fluids, has not acquainted you with this major aspect of vehicle safety research.
"or your (sic) a chav and drive like an tw*t.
No. This problem has affected all types of motorists over the years and Jaguar found out the hard way when testing those new fangled disc brakes and finding that the high temperatures, generated at the caliper, were leading to spongy brakes, caused by compressible water vapour which formed in the hydraulic brake system.
 
watchmender I think you should read my post slowly and then get your head out of your back passage. I think my piece about Ford taking samples of brake fluid from customers cars might be similar to what you are bleating on about, with regards to the boys in blue they are a set of wan*ers and generally have no technical knowledge.
I agree that brake fluid needs to be changed on a regular basis. I will now give you some inside knowledge, brake pads now that they are asbestos free will generally have a layer of insulating material next to the back plate.
With regards to Jaguar's brake gassing problem can you tell me what grade of brake fluid they used, was it dot 3 or dot?.
over to you.
 
"get your head out of your back passage."
"what you are bleating on about
"the boys in blue they are a set of wan*ers

Sorry Caesar, I do hope that your original enquiry did generate some helpful comments and that you have been guided towards areas which might help with your braking problem. Unfortunately, the vulgarity and childishness of some comments, as above, do indicate that there are those whose intentions are far from helpful.
 
I notice nobodys suggested pinching off the flex hoses to find out if its the master cylinder or a brake that is causing the soft pedal.
 
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